Atomic layer etching processes using sequential, self-limiting thermal reactions comprising oxidation and fluorination

ABSTRACT

The invention includes a method of promoting atomic layer etching (ALE) of a surface. In certain embodiments, the method comprises contacting a solid substrate comprising a first metal compound with an oxidant, optionally contacting the solid substrate with a second metal compound, and then contacting the modified solid substrate with a fluorinating agent, whereby ALE of the solid substrate is promoted.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/456,944, filed Feb. 9, 2017, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The continued miniaturization of advanced semiconductor devices requires atomic layer control in both growth and etching processes. Atomic layer deposition (ALD) and atomic layer etching (ALE) techniques can provide the necessary atomic level precision. ALD techniques have been developed for a wide range of materials over the past few decades, and have been extensively adapted by the semiconductor industry. In contrast, the need for ALE techniques has emerged more recently, and ALE methods are still in an early stage of development.

Initial plasma ALE methods have been based on surface activation by halogenation, followed by ion bombardment to remove surface material. Plasma processes have been developed for a variety of materials including Si, compound semiconductors, metal oxides, and carbon materials. The plasma ALE method can further achieve anisotropic etching.

Thermal ALE techniques have also been demonstrated with fluorination and ligand-exchange reactions. Thermal ALE methods have been developed for Al₂O₃, HfO₂, ZrO₂, AlN and AlF₃. Thermal ALE is capable of providing isotropic etching. In contrast, materials with volatile metal fluorides do not have thermal ALE pathways using fluorination and ligand-exchange reactions because their fluorides are gases. Other materials, such as elemental metals, may fluorinate readily and produce fluoride layers that are too thick for ALE. For these materials, alternative pathways are required for controlled ALE.

There remains a need in the art for methods that allow for ALE to be used in materials resistant to known plasma ALE and thermal ALE techniques. The present invention addresses and meets this need.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the invention provides a method of promoting atomic layer etching (ALE) on a solid substrate comprising a first metal compound.

In certain embodiments, the method comprises contacting at least a portion of the surface of the solid substrate with an oxidant, whereby a first metal oxide is formed on the portion of surface of the solid substrate contacted with the oxidant. In other embodiments, the method comprises contacting at least a portion of the surface of the first metal oxide with a fluorinating agent, whereby a volatile metal fluoride is formed; whereby ALE of the solid substrate is promoted.

In certain embodiments, the method comprises contacting at least a portion of the surface of the solid substrate with an oxidant, whereby a first metal oxide is formed on the portion of surface of the solid substrate contacted with the oxidant. In other embodiments, the method comprises contacting at least a portion of the surface of the first metal oxide with a second metal compound, whereby a second metal oxide is formed on the portion of surface of the first metal oxide contacted with the second metal compound and whereby a volatile first metal species is further formed. In yet other embodiments, the method comprises contacting at least a portion of the surface of the second metal oxide with a fluorinating agent, whereby a volatile metal fluoride is formed; whereby ALE of the solid substrate is promoted.

In certain embodiments, the sequential steps of the method are repeated one or more times, wherein each repetition of the sequential steps etches the solid substrate at a rate of about 0.1 Å to about 3.0 Å per cycle of sequential steps.

In certain embodiments, the first metal compound comprises at least one selected from the group consisting of a metal-Group III, metal-Group IV, metal-Group V, metal-Group VI, elemental metal, metal alloy, hybrid organic-inorganic material, and any combinations of thereof. In other embodiments, the first metal compound comprises at least one selected from the group consisting of metal boride, metal carbide, metal silicide, metal germanide, metal nitride, metal phosphide, metal arsenide, metal antimonide, metal sulfide, metal selenide, metal telluride, an elemental metal, metal alloy, hybrid organic-inorganic material, and any combinations thereof.

In certain embodiments, the first metal compound is capable of forming a volatile metal fluoride. In other embodiments, the first metal fluoride has a melting point of less than 200° C. at 1 atm pressure. In other embodiments, the first metal compound comprises a species selected from the group consisting of Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr, Pt, B, Si, Ge, As, Sb, and Te.

In certain embodiments, the second metal compound comprises a metal species capable of forming a stable metal oxide species on the surface of the first metal oxide. In other embodiments, the first metal oxide comprises a metal species capable of forming at least one first metal species selected from the group consisting of a volatile metal oxyfluoride, a volatile metal chloride, a volatile metal oxychloride, a volatile metal bromide, a volatile oxybromide, a volatile metal iodide, a volatile oxyiodide, and a volatile metal organic compound. In certain embodiments, the first metal species has a melting point of less than 200° C. at 1 atm pressure.

In other embodiments, the second metal compound comprises a metal species capable of forming at least one second metal species selected from the group consisting of a volatile metal fluoride, volatile metal oxyfluoride, volatile metal chloride, volatile metal oxychloride, volatile metal bromide, volatile metal oxybromide, volatile metal iodide, and volatile metal oxyiodide. In certain embodiments, the second metal species has a melting point of less than 200° C. at 1 atm pressure.

In yet other embodiments, first metal compound comprises a species selected from the group consisting of Al, Hf, Zr, Fe, Ni, Co, Mn, Mg, Rh, Ga, In, Zn, Pb, Cu, Cd, Hg, Ca, Li, Ag, Sn, Y, Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr, Pt, B, Si, Ge, As, Sb, and Te. In yet other embodiments, the second metal compound is at least one selected from BCl₃, B(CH₃)₃, BBr₃, BI₃, and TiCl₄. In other embodiments, the second metal oxide is B₂O₃ or TiO₂.

In certain embodiments, the oxidant is a gaseous oxidant selected from the group consisting of O₂, H₂O₂, O₃, oxygen plasma, reactive oxygen radical species, NO₂ plasma, N₂O plasma, NO plasma, HNO₃ plasma and SO₂ plasma.

In certain embodiments, the fluorinating agent is a gaseous fluorinating agent selected from the group consisting of HF, SF₄, XeF₂, F₂, and SF₆ plasma.

In certain embodiments, the method is used for at least one process selected from patterning a surface of the substrate, cleaning a surface of the substrate, creating pores in a surface of the substrate, enlarging pores in a porous surface of the substrate, smoothing a surface of the substrate, removing defects from a surface of the substrate, removing damaged surface layers of the substrate and removing unwanted clusters or islands from a surface of the substrate.

In certain embodiments, the method is used to produce a substrate film having a thickness of less than about 10 nm by first depositing a film having a thickness greater than about 10 nm and then etching the film by performing the sequential steps of the method one or more times.

In certain embodiments, the solid substrate is formed using atomic layer deposition (ALD).

BRIEF DESCRIPTION OF THE FIGURES

For the purpose of illustrating the invention, depicted in the drawings are certain embodiments of the invention. However, the invention is not limited to the precise arrangements and instrumentalities of the embodiments depicted in the drawings.

FIG. 1A is a schematic for TiN ALE using O₃ and HF as the reactants according to an exemplary embodiment of the invention. O₃ oxidizes the TiN substrate to form a TiO₂ layer on the surface. Subsequently, HF removes the TiO₂ layer by forming volatile TiF₄ and H₂O reaction products.

FIG. 1B is a scheme showing an oxidation, conversion and fluorination reaction, according to an exemplary embodiment of the invention. Step (A) shows oxidation of W using O₂/O₃. Step (B) shows conversion of W to WO₃ using BCl₃. Step (C) shows fluorination of B₂O₃ by HF to form volatile BF₃ and H₂O.

FIG. 2 is a schematic showing an ellipsometer beam interacting with a film stack comprised of WO₃, W, Al₂O₃, SiO₂, and underlying Si substrate.

FIG. 3 is a graph showing mass changes during 10 consecutive HF exposures on TiO₂ ALD film at 200, 250, and 300° C.

FIG. 4 is a graph showing X-ray reflectivity and spectroscopic ellipsometry measurements of TiN film thickness vs. number of O₃ and HF reaction cycles at 250° C.

FIG. 5A is a graph of etch rates for TiN vs. O₃ exposure time with constant HF exposure time of 1.0 s.

FIG. 5B is a graph of etch rates for TiN vs. O₃ exposure time with constant HF exposure time of 3.0 s.

FIG. 6 is a graph showing film thickness vs. number of O₃ and HF reaction cycles at 250° C. for Al₂O₃, HfO₂, ZrO₂, SiO₂, Si₃N₄, and TiN.

FIG. 7 is a graph showing X-ray reflectivity scans vs. number of O₃ and HF reaction cycles on TiN films at 250° C.

FIG. 8 is a graph showing etch rates for TiN ALE vs. temperature using O₃ and HF as the reactants.

FIG. 9A is a graph showing film thickness for (a, upper line) initial TiN film and (b, lower line) after 200 TiN ALE cycles at 250° C. for different locations in the reactor.

FIG. 9B is a graph showing film thickness for (a, upper line) initial TiN film and (b, lower line) after 200 TiN ALE cycles at 350° C. for different locations in the reactor.

FIG. 10 is a graph showing X-ray reflectivity and spectroscopic ellipsometry measurements of TiN film thickness vs. number of H₂O₂ and HF reaction cycles at 250° C.

FIG. 11 is a graph showing X-ray reflectivity scans vs. number of H₂O₂ and HF reaction cycles on TiN films at 250° C.

FIG. 12 is a graph showing WO₃ thickness vs. number of cycles showing WO₃ ALE at 207° C. using BCl₃ and HF as reactants. The W film under the WO₃ layer acts as an etch stop.

FIG. 13 is a graph showing WO₃ thickness vs. number of cycles showing WO₃ ALE at 207° C. using BCl₃ and HF as reactants. Etch rate during WO₃ ALE is 4.18 Å/cycle.

FIG. 14 is a graph showing B₂O₃ thickness vs. number of HF exposures showing spontaneous etching of B₂O₃ film at 207° C. HF exposure was 100 mTorr s, and B₂O₃ etch rate is ˜2 Å per HF exposure.

FIGS. 15A-15B are graphs showing WO₃ etch rate vs. reactant exposure during WO₃ ALE at 207° C. FIG. 15A is a graph showing WO₃ etch rate as BCl₃ exposure was varied with HF exposure held at 200 mTorr s. FIG. 15B is a graph showing WO₃ etch rate as HF exposure was varied with BCl₃ exposure held at 327 mTorr s.

FIG. 16 is a graph showing WO₃ thickness vs. number of half-cycles during WO₃ ALE at 207° C. under self-limiting conditions.

FIG. 17 is a graph showing an analysis of WO₃ thickness change after BCl₃ exposure and HF exposure during WO₃ ALE at 207° C.

FIG. 18 is a graph showing WO₃ thickness vs. number of cycles for WO₃ ALE at 128° C., 160° C., 196° C. and 207° C.

FIG. 19 is an Arrhenius plot of temperature-dependent etch rates for WO₃ ALE. Slope of the Arrhenius plot yields an activation barrier of 8.6 kcal/mol.

FIG. 20 is a graph of W thickness vs. number of cycles during W ALE at 207° C. using O₂/O₃, BCl₃, and HF as reactants. Etch rate during W ALE is 2.56 Å/cycle.

FIGS. 21A-21B are graphs showing WO₃ and W thicknesses vs. number of half-cycles during W ALE under self-limiting conditions using O₂/O₃, BCl₃, and HF as reactants at 207° C. FIG. 21A shows WO₃ thickness showing oscillation of WO₃ thickness after O₂/O₃ exposure and BCl₃/HF reaction. FIG. 21B shows W thickness showing linear reduction vs. number of half-cycles with an etch rate of 2.44 Å/cycle.

FIG. 22 is a graph showing enlargement of WO₃ thickness vs. number of half-cycles showing increase and decrease of WO₃ thickness after O₂/O₃ exposure and BCl₃/HF reaction.

FIG. 23 is a graph showing W etch rate vs. O₂/O₃ exposure during W ALE at 207° C. BCl₃ and HF exposures were held at 500 and 2800 mTorr s, respectively.

FIGS. 24A-24B are graphs showing W etch rate vs. reactant exposure during W ALE. FIG. 24A is a graph showing W etch rate as BCl₃ exposure was varied with HF and O₂/O₃ exposures held at 2800 and 3150 mTorr s, respectively. FIG. 24B is a graph showing W etch rate as HF exposure was varied with BCl₃ and O₂/O₃ exposures held at 500 and 3150 mTorr s, respectively.

FIG. 25 is a graph showing WO₃ thickness vs. number of O₂/O₃ exposures for initial W ALD film. Each O₂/O₃ exposure was 70 mTorr for 45 s.

FIGS. 26A-26B are graphs showing WO₃ removal after W ALE using BCl₃ and HF as reactants. FIG. 26A shows WO₃ thickness vs. number of cycles showing reduction of WO₃ thickness to limiting value of ˜3 Å. FIG. 26B shows W thickness vs. number of cycles showing that W thickness remains nearly constant during WO₃ removal.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the unexpected discovery of novel methods of promoting and/or enhancing atomic layer etching (ALE) of a metal-containing surface using sequential, self-limiting thermal reactions. In certain embodiments, the metal surface is contacted with an oxidant, thereby forming a metal oxide species, which is optionally contacted with a conversion reactant, thereby forming a conversion oxide species. The modified surface is then contacted with a fluorinating agent, thereby forming a volatile fluoride species that can be removed from the surface through ALE.

Disclosure

Thermal ALE Based on Oxidation & Fluorination Reactions

Many materials are not easily etched using sequential fluorination and ligand-exchange reactions. For example, TiN was not etched by sequential exposures of HF and various metal precursors such as Sn(acac)₂, Al(CH₃)₃, AlCl (CH₃)₂ or SiCl₄. Either the TiN is not fluorinated by HF or the ligand-exchange reactions with the metal precursors do not produce stable and volatile reaction products. However, alternative chemical strategies can be employed for TiN ALE.

New TiN ALE chemistry can be defined using oxidation and fluorination reactions. For example, the surface of TiN can be converted to a TiO₂ surface layer by an oxidation reaction with either H₂O₂ or O₃ according to: TiN+3H₂O₂→TiO₂+NO+3H₂O  (1) —or— TiN+3O₃→TiO₂+NO+3O₂  (2) These reactions are thermochemically favorable and are predicted to be spontaneous based on their negative ΔG values. The H₂O₂ reaction given by Equation 1 has a predicted Gibbs Free energy change of ΔG=−212 kcal/mole at 250° C. The O₃ reaction given by Equation 2 has a predicted Gibbs Free energy change of ΔG=−242 kcal/mole at 250° C.

These oxidation reactions form a self-passivating oxide layer on the nitride substrate. The oxide layer is self-passivating because the oxide layer serves as a diffusion barrier to subsequent oxidation. In addition, the oxide layer is also self-passivating because the oxide layer has a Pilling-Bedworth ratio >1. For TiN oxidizing to TiO₂, the Pilling-Bedworth ratio is 1.65 The metal oxide expands relative to the metal nitride and forms a continuous metal oxide film on the metal nitride. There also may be compressive stress in the metal oxide film that inhibits further oxidation.

Following the conversion of TiN to TiO₂, the TiO₂ can then be etched by a fluorination reaction with HF according to: TiO₂+4HF→TiF₄+2H₂O  (3) Note that this reaction has a small positive Gibbs Free energy change of ΔG=+6.3 kcal/mole at 250° C. However, the reaction is known to occur as shown in Example 1. Thermochemical calculations are not always accurate predictors of a reaction when the ΔG value is small. Reactions are not usually run under equilibrium conditions. The volatile products of a reaction are allowed to escape under vacuum conditions. In addition, the surface reaction products may not have heats of formation that are the same as bulk species.

After removing the TiO₂ layer, the underlying TiN is not spontaneously etched by HF. Instead, the TiN is believed to be fluorinated according to: TiN+3HF→TiF₃+NH₃  (4) This reaction has a predicted Gibbs Free energy change of ΔG=−37 kcal/mole at 250° C. The formation of a passivating TiF₃ layer acts as a barrier to further fluorination.

The above reactions demonstrate that TiN ALE can be performed using sequential oxidation and fluorination reactions. In this thermal ALE process, the fluorination reaction in Equation 3 is responsible for etching because TiF₄ is volatile. In contrast, the previous thermal ALE processes based on fluorination and ligand-exchange reactions required a stable metal fluoride to be formed during the fluorination reaction. The metal fluoride was then removed by the ligand-exchange reaction.

Other metal nitrides could be etched using a similar procedure. Examples of these metal nitrides include TaN, NbN and BN. These metal nitrides all have volatile metal fluorides. These metal nitrides would not be expected to have a self-limiting fluorination reaction using the earlier thermal ALE procedure based on fluorination and ligand-exchange. However, using the new strategy based on oxidation and fluorination reactions, a self-limiting oxidation can first convert the metal nitride to the self-passivating metal oxide. Then HF can remove the metal oxide by spontaneous etching of the metal oxide because the metal fluoride is volatile.

The oxidation and fluorination reactions for TaN ALE are given below. O₃ is used as the oxidation reagent. TaN+7/2O₃→½Ta₂O₅+NO+7/2O₂  (5) ½Ta₂O₅+5HF→TaF₅(g)+5/2H₂O  (6)

Likewise, the oxidation and fluorination reactions for BN ALE are shown below. O₃ is again used as the oxidation reagent. BN+5/2O₃→½B₂O₃+5/2O₂+NO  (7) ½B₂O₃+3HF→BF₃+3/2H₂O  (8)

For TaN and BN ALE, the oxidation reaction again converts the metal nitride to the self-passivating metal oxide. The oxide layer is self-passivating because the oxide layer serves as a diffusion barrier to subsequent oxidation. In addition, the oxide layer is also self-passivating because the oxide layer has a Pilling-Bedworth ratio >1. For TaN oxidized to Ta₂O₅, the Pilling-Bedworth ratio is 1.89. For BN oxidized to B₂O₃, the Pilling-Bedworth ratio is 1.19. The metal oxide expands relative to the metal nitride and forms a continuous metal oxide film on the metal nitride. There also may be compressive stress in the metal oxide film that acts to inhibit further oxidation.

In addition, the oxidation reaction can change the oxidation state of the metal in the metal nitride. The oxidation of TiN to TiO₂ changes the oxidation state of Ti from 3+ to 4+. The oxidation of TaN to Ta₂O₅ changes the oxidation state of Ta from 3+ to 5+. These changes in oxidation state are important to establish the correct oxidation state for the volatile metal fluoride produced by the HF reaction. The oxidation state of Ti in TiF₄ is 4+. The oxidation state of Ta in TaF₅ is 5+. This change is oxidation state is important to produce a volatile fluoride from the metal oxide. Without intending to be limited to any particular theory, it is believed that fluorination of the metal oxide does not involve a change in the oxidation state.

The oxidation of BN to B₂O₃ does not change the oxidation state of B. The oxidation state of B in both BN and B₂O₃ is 3+. A change in oxidation state is not necessary because the oxidation state of the volatile BF₃ reaction product is also 3+. However, this lack of change in the oxidation state also may lead to the spontaneous etching of BN by HF. The BN underlying the B₂O₃ may not act as an etch stop during the HF exposure. In non-limiting embodiments, a change in oxidation state during the oxidation reaction to the oxidation state of the volatile fluoride yields an effective ALE process.

The methods can be further expanded to additional materials, as discussed in Example 4.

Thermal ALE Based on Oxidation, Conversion & Fluorination Reactions

Many metal oxides do not yield a volatile metal fluoride upon reaction with HF. However, some of these metal oxides can first be converted to an alternative metal oxide. The alternative metal oxide then may display a volatile metal fluoride upon reaction with HF.

In addition, some elemental metals do not form a stable metal fluoride because the metal fluoride is highly volatile. To avoid the spontaneous etching of the elemental metal during fluorination, some elemental metals can first be oxidized to the metal oxide. The metal oxide can then be etched with atomic layer control using fluorination and ligand-exchange reactions.

Alternatively, if the metal oxide does not undergo ALE by fluorination and ligand-exchange reactions, the metal oxide may be first converted to an alternative metal oxide. This alternative metal oxide may then undergo ALE by fluorination and ligand-exchange reactions or may display a volatile metal fluoride upon reaction with HF.

The introduction of the conversion reaction sidesteps problems that can prevent the definition of effective ALE procedures. The conversion reaction can convert the initial material to an alternative material that avoids the problems. The introduction of the conversion reaction can lead to a three-step etch sequence for ALE based on oxidation, conversion and fluorination reactions.

W ALE is a model system for thermal ALE of an elemental metal based on oxidation, conversion and fluorination reactions. Tungsten (W) has a very volatile fluoride, WF₆. Fluorination with various fluorination precursors does not lead to a self-limiting metal fluoride. Instead, fluorination with fluorination precursors, such as XeF₂, leads to a volatile WF₆ reaction product and spontaneous etching.

To avoid this spontaneous etching, W can first be oxidized to WO₃ using O₃ or an O₂ plasma: W+3O₃(g)→WO₃+3O₂(g)  (9)

This reaction with O₃ is thermochemically favorable with a predicted Gibbs Free energy change of ΔG=−211 kcal/mole at 250° C. The oxidation reaction forms a self-passivating oxide layer on the W substrate. The oxide layer is self-passivating because the oxide layer serves as a diffusion barrier to subsequent oxidation. In addition, the oxide layer is also self-passivating because the oxide layer has a Pilling-Bedworth ratio >1. For W oxidizing to WO₃, the Pilling-Bedworth ratio is 3.39. The WO₃ expands relative to the tungsten metal and forms a continuous WO₃ film. There also may be compressive stress in the WO₃ film that inhibits further oxidation.

WO₃ can then lead to etching pathways. However, exposure of WO₃ to HF does not form volatile WF₆ based on the reaction: WO₃+6HF→WF+3H₂O  (10) This reaction is not thermochemically favorable because the predicted Gibbs Free energy change is ΔG=+35 kcal/mole at 250° C. Although WO₃ does not produce volatile products during HF exposures, other oxides may lead to volatile products. If WO₃ can be converted to these other oxides, then etching pathways may be possible.

Fortunately, WO₃ can be converted to another metal oxide. Following W oxidation to WO₃, the WO₃ can be converted to B₂O₃. This conversion reaction can occur by reaction with BCl₃: WO₃+2BCl₃→B₂O₃+WCl₆  (11)

This reaction is thermochemically favorable with a predicted Gibbs Free energy change of ΔG=−6 kcal/mole at 250° C.

After the conversion of WO₃ to B₂O₃ using BCl₃, B₂O₃ can be etched with HF according to the reaction: B₂O₃+6HF→2BF₃(g)+3H₂O  (12) This reaction is favorable as demonstrated in Example 2. The predicted Gibbs Free energy change is ΔG=−17 kcal/mole at 250° C. The conversion of WO₃ to B₂O₃ by reaction with BCl₃ is necessary to produce a volatile BF₃ product in the reaction given by Equation 12.

The conversion of various metal oxides to B₂O₃ can occur if B₂O₃ is more stable than the initial metal oxide. This conversion pathway to B₂O₃ will be important for a variety of metal oxides that do not have effective ALE pathways. There may be no ALE pathways because the metal oxide may etch spontaneously with no atomic layer control upon exposure to HF. Another possibility that would prevent ALE is that the metal oxide may form a metal fluoride that may not undergo a facile ligand-exchange reaction. The conversion to B₂O₃ may solve either of these two difficulties.

The conversion of metal oxides to B₂O₃ may provide a pathway to ALE for various other metal oxides. For example, possible conversion reactions with BCl₃ at 200° C. for a variety of metal oxides are given below:

ΔG (kcal at 200° C.) 3WO₃ + 2BCl₃(g) → B₂O₃ + 3WO₂Cl₂(g) −23 3MoO₃ + 2BCl₃(g) → B₂O₃ + 3MoO₂Cl₂(g) −60 3CrO₃ + 2BCl₃(g) → B₂O₃ + 3CrO₂Cl₂(g) −96 1.5GeO₂ + 2BCl₃(g) → B₂O₃ + 1.5GeCl₄(g) −76 1.5SnO₂ + 2BCl₃(g) → B₂O₃ + 1.5SnCl₄(g) −71 These reactions all have negative ΔG values at 200° C. These negative ΔG values indicate that these reactions are all predicted to be spontaneous.

The thermal ALE process based on oxidation, conversion and fluorination reactions is general. This procedure may apply for the ALE of elemental metals and metal compounds with volatile fluorides that cannot be etched with atomic layer control using fluorination and ligand-exchange reactions because the fluorination reaction is spontaneous. In addition, the fluorination reaction is very robust for elemental metals. Even for elemental metals with stable, non-volatile fluorides, initial oxidation of the elemental metals may be desirable to prevent the large heats of fluorination from leading to thick fluoride layers.

A variety of elemental metals could be etched with atomic layer control using oxidation, conversion and fluorination reactions including Mo, Ir, Re, Ru, Ta and Nb. These metals all have volatile fluorides and would not display self-limiting fluorination reactions. Fortunately, these metals can first be oxidized and then converted to B₂O₃. For example, elemental Mo can be oxidized using ozone according to: Mo+O₃→MoO₃  (13)

This reaction is predicted to be spontaneous based on a predicted Gibbs Free energy change of ΔG=−188 kcal/mole at 250° C. MoO₃ can then be converted to B₂O₃ by reaction with BCl₃ according to: MoO₃+2BCl₃→B₂O₃+MoCl₆  (14)

This reaction is predicted to be spontaneous based on a predicted Gibbs Free energy change of ΔG=−16 kcal/mole at 250° C. Subsequently, B₂O₃ could be removed with HF according to Equation 12.

A variety of metal nitrides could also be etched using oxidation, conversion and fluorination reactions including WN, MoN, TaN, NbN. These metal nitrides all have volatile fluorides and may not display self-limiting fluorination reactions. For example, WN could first be oxidized using ozone according to: WN+4/3O₃→WO₃+NO  (15)

This reaction is predicted to be spontaneous based on a predicted Gibbs Free energy change of ΔG=−390 kcal/mole at 250° C. WO₃ could then be converted to B₂O₃ by reaction with BCl₃ according to Equation 11. Subsequently, B₂O₃ could be removed with HF according to Equation 12.

Thermal ALE based on oxidation, conversion and fluorination reactions could also be employed for metal sulfides and metal selenides of metals that have volatile fluorides. For example, MoS₂, WS₂, MoSe₂ and WSe₂ are metal sulfides and metal selenides composed of metals, Mo and W, that have volatile fluorides. These metal sulfides and metal selenides are also important two-dimensional (2D) materials.

These metal sulfides and metal selenides could be etched with atomic layer control using oxidation, conversion and fluorination reactions. For example, WS₂ could first be oxidized using ozone according to: WS₂+3O₃→WO₃+2SO₃  (16)

This reaction is predicted to be spontaneous based on a predicted Gibbs Free energy change of ΔG=−406 kcal/mole. Subsequently, WO₃ could be converted to B₂O₃ by reaction with BCl₃ according to Equation 11. Then the B₂O₃ could be removed by reaction with HF according to Equation 12.

In another example, MoS₂ could first be oxidized using ozone according to: MoS₂+3O₃→MoO₃+2SO₃  (17)

This reaction is predicted to be spontaneous based on a predicted Gibbs Free energy change of ΔG=−379 kcal/mole. Subsequently, MoO₃ could be converted to B₂O₃ by reaction with BCl₃ according to Equation 14. Then the B₂O₃ could be removed by reaction with HF according to Equation 12.

The methods can be further expanded to additional materials, as discussed in Example 7.

Methods

The invention provides a method of promoting atomic layer etching (ALE) on a solid substrate comprising a first metal compound. In certain embodiments, the method comprises (a) contacting at least a portion of the surface of the solid substrate with an oxidant, whereby a first metal oxide is formed on the portion of surface of the solid substrate contacted with the oxidant. In other embodiments, the method comprises (b) optionally contacting at least a portion of the surface of the first metal oxide with a second metal compound, whereby a volatile first metal species, and a second metal oxide are formed. In yet other embodiments, the method comprises contacting the surface of the first metal oxide formed in (a), and/or the second metal oxide formed in (b), with a fluorinating agent, whereby a volatile metal fluoride is formed and ALE of the solid substrate is promoted.

In certain embodiments, the first metal compound comprises at least one selected from the group consisting of a metal-Group III, metal-Group IV, metal-Group V, metal-Group VI, elemental metal, metal alloy, hybrid organic-inorganic material, and any combinations of thereof. In certain embodiments, the first metal compound comprises at least one selected from the group consisting of a metal boride, metal carbide, metal silicide, metal germanide, metal nitride, metal phosphide, metal arsenide, metal antimonide, metal sulfide, metal selenide, metal telluride, an elemental metal, metal alloy, hybrid organic-inorganic material; and any combinations thereof.

In certain embodiments wherein step (b) is not carried out, the first metal compound comprises a metal species capable of forming a metal fluoride upon reaction with the fluorinating agent, wherein the metal fluoride is volatile under the reaction conditions. In other embodiments, the first metal compound comprises a metal species capable of forming a metal fluoride with a melting point of less than 100° C. at 1 atm pressure. In yet other embodiments, the first metal compound comprises a metal species capable of forming a metal fluoride with a melting point of less than 200° C. at 1 atm pressure. In yet other embodiments, the first metal compound comprises a metal species capable of forming a metal fluoride with a melting point of less than 250° C. at 1 atm pressure.

In certain embodiments wherein step (b) is not carried out, the first metal compound comprises at least one selected from the group consisting of Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr and Pt. In certain embodiments wherein step (b) is not carried out, the first metal compound comprises at least one selected from the group consisting of B, Si, Ge, As, Sb and Te.

In certain embodiments wherein step (b) is carried out, the first metal compound comprises a metal species capable of forming at least one selected from the group consisting of a volatile metal oxyfluoride, a volatile metal chloride, a volatile metal oxychloride, a volatile metal bromide, a volatile oxybromide, a volatile metal iodide, a volatile oxyiodide, and a volatile metal organic compound.

In other embodiments wherein step (b) is carried out, the first metal compound comprises a metal species capable of forming a metal fluoride metal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 100° C. at 1 atm pressure. In yet other embodiments, the first metal compound comprises a metal species capable of forming a metal fluoridemetal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 200° C. at 1 atm pressure. In yet other embodiments, the first metal compound comprises a metal species capable of forming a metal fluoridemetal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 250° C. at 1 atm pressure.

In certain embodiments wherein step (b) is carried out, the first metal compound comprises at least one metal capable of forming a volatile fluoride selected from the group consisting of Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr and Pt. In other wherein step (b) is carried out, the first metal compound comprises at least one metal that does not form a volatile fluoride selected from the group consisting of Al, Hf, Zr, Fe, Ni, Co, Mn, Mg, Rh, Cr, Ga, In, Zn, Pb, Cu, Cd, Hg, Ca, Li, Ag, Sn and Y.

In certain embodiments wherein step (b) is carried out, the second metal compound comprises a metal species capable of forming a stable metal oxide species on the surface of the solid substrate. In other embodiments wherein step (b) is carried out, the second metal compound comprises a metal species capable of forming at least one selected from the group consisting of a volatile metal fluoride, volatile metal oxyfluoride, volatile metal chloride, volatile metal oxychloride, volatile metal bromide, volatile metal oxybromide, volatile metal iodide and volatile metal oxyiodide. In other embodiments wherein step (b) is carried out, the second metal compound comprises a metal species capable of forming a metal fluoride metal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 100° C. at 1 atm pressure. In yet other embodiments, the second metal compound comprises a metal species capable of forming a metal fluoridemetal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 200° C. at 1 atm pressure. In yet other embodiments, the second metal compound comprises a metal species capable of forming a metal fluoridemetal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide or metal oxyiodide with a melting point of less than 250° C. at 1 atm pressure.

In certain embodiments wherein step (b) is carried out, the second metal compound comprises a metal species capable of forming at least one selected from the group consisting of a volatile metal oxyfluoride, volatile metal chloride, volatile metal oxychloride, volatile metal bromide, volatile metal oxybromide, volatile metal iodide and volatile metal oxyiodide upon reaction with the first metal oxide.

In certain embodiments wherein step (b) is carried out, the second metal compound is at least one selected from BCl₃, B(CH₃)₃, BBr₃, BI₃, and TiCl₄. In other embodiments wherein step (b) is carried out, the second metal oxide is B₂O₃ or TiO₂.

In certain embodiments the oxidant is a gaseous oxidant. In other embodiments, the oxidant is selected from the group consisting of O₂, H₂O₂, O₃, oxygen plasma, reactive oxygen radical species, NO₂ plasma, N₂O plasma, NO plasma, HNO₃ plasma and SO₂ plasma.

In certain embodiments, the fluorinating agent is a gaseous fluorinating agent. In other embodiments, the fluorinating agent is at least one compound selected from the group consisting of HF, SF₄, XeF₂, F₂, and SF₆ plasma. In other embodiments, the fluorinating agent is any other fluorine containing plasma known in the art.

In certain embodiments, steps (a) and (c) [or (a), (b), and (c)] can be repeated one or more times. In certain embodiments, the method allows for etching of the solid substrate at a rate of about 0.1 Å per cycle to about 3.0 Å per cycle of steps (a)-(c).

In certain embodiments, the solid substrate is deposited on an inert substrate material. In other embodiments, the inert substrate material comprises silicon. In other embodiments, the inert substrate material comprises a silicon wafer.

In certain embodiments, the solid substrate is formed using atomic layer deposition.

In certain embodiments, the method of the invention is used for at least one purpose selected from the group consisting of patterning the solid substrate, cleaning the solid substrate, creating pores in the solid substrate and surface smoothing.

The invention further provides a method of smoothing the surface of a solid substrate, wherein the solid substrate comprises a first metal compound. Smoothing of surfaces is of interest in the semiconductor industry. For example, smoothing can be used to obtain damage-free layers. In contrast, sputtering can be used to remove some materials, but leaves a rough, damaged surface. ALE can be used to remove the damaged layer and smooth the surface to produce a “damage-free surface.”

Surface smoothing can also be used to obtain very high quality ultrathin films. For example, high quality ultrathin films can be produced by a “deposit/etch back” strategy by depositing a thicker film and then etching back to a thinner film. In a non-limiting embodiment, nucleation effects can lead to roughness in the ultrathin deposited film; once a continuous and pinhole-free thicker film is formed, ALE can etch this film back and obtain a smoother surface than would have been produced by growing to this ultrathin thickness.

In certain embodiments, the invention further provides a method of removing clusters, islands and other raised defects formed by atomic layer deposition (ALD) on a surface. These clusters and islands are the result of nucleation delays during ALD. The clusters and islands may be composed of any metal compounds disclosed elsewhere herein. In area selective deposition, growth is favored on some surfaces and not favored on other surfaces. These clusters and islands are not desired on the unfavored surfaces. The clusters and islands can be etched away through the ALE methods of the invention to improve the selectivity of the ALD on the preferred surface.

In certain embodiments, each cycle of steps (a)-(c) is self-limiting. In other embodiments, each cycle of steps (a)-(c) allows for about the same etch rate and/or mass loss of the solid substrate. In yet other embodiments, the resulting etched solid substrate has about the same or lower surface roughness as the starting solid substrate. In yet other embodiments, the density of the etched solid substrate is about the same as of the starting solid substrate. In yet other embodiments, the refractive index of the etched solid substrate is about the same as of the initial solid substrate.

In certain embodiments, the solid substrate comprises at least one additional metal compound, and ALE of the first metal compound is selective over ALE of the at least one additional metal compound. In other embodiments, the ALE rate is (a) controlled by the solid substrate temperature, and/or (b) enhanced by pressure of the oxidation and/or fluorination reagent and/or the second metal precursor. In yet other embodiments, the ALE is performed using a spatial ALE technique.

In certain embodiments, the surface of the starting solid substrate is at least partially masked, whereby only a portion of the surface of the solid substrate is exposed. In other embodiments, the surface of the starting solid substrate is at least partially masked with a photoresist. In yet other embodiments, the solid substrate is further patterned using a technique selected from the group consisting of monolayer self-assembling, nano-imprint and stamping.

In certain embodiments, the method comprises applying directionally to a fraction of the surface of the solid substrate an agent that promotes removal of at least a fraction of any surface species that results from the ALE reaction and is bound to, and/or adsorbed on, a fraction of the surface of the solid substrate. In other embodiments, the applying is performed concomitantly with the ALE reaction and/or in between steps of the ALE reaction.

In certain embodiments, the agent is at least one selected from the group consisting of a radical, photon, ion, electron, catalyst, and molecular adsorbate. In other embodiments, the agent is generated by a plasma. In yet other embodiments, the agent comprises a radical and the plasma comprises a H₂ plasma. In yet other embodiments, the agent comprises a photon and the plasma comprises an Ar plasma. In yet other embodiments, the agent comprises an alkali metal catalyst, which is formed on the solid substrate from an alkali metal molecular precursor. In yet other embodiments, wherein the agent comprises a molecular precursor that adsorbs on the solid substrate and displaces other surface-adsorbed species that limit progress of the thermal ALE.

Compositions

The invention provides a solid substrate obtained according to the methods of the invention. The invention further provides a 3D structure with surface features obtained according to the methods of the invention. The invention further provides a patterned solid substrate obtained according to the methods of the invention. The invention further provides a smoothened solid substrate obtained according to the methods of the invention. The invention further provides a nanodevice fabricated using the methods of the invention.

Definitions

As used herein, each of the following terms has the meaning associated with it in this section.

As used herein, unless defined otherwise, all technical and scientific terms generally have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Generally, the nomenclature used herein and the laboratory procedures in surface chemistry are those well-known and commonly employed in the art.

As used herein, the articles “a” and “an” refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.

As used herein, the term “about” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which it is used. As used herein, “about” when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20% or ±10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

As used herein, the term “ALD” refers to atomic layer deposition, which is a thin film deposition method. In certain embodiments, the term “thin” refers to a range of thickness from about 0.1 nm to about 300 nm. ALD uses a self-limiting reaction sequence that deposits films in discrete steps limited by surface site chemical reactions. It produces continuous films with atomically controlled thicknesses, high conformality, and typically pinhole-free and atomically smooth surfaces. These are essential properties as design constraints push device technologies to ever smaller sizes. In certain embodiments, molecular layer deposition (MLD) is a non-limiting example of ALD. In other embodiments, materials prepared using ALD include materials prepared using MLD. In yet other embodiments, MLD comprises depositing an alkoxide polymer on a substrate. In yet other embodiments, MLD comprises generating a polymer by alternating reactions of chemicals selected from a first and a second group; wherein the first group includes, but is not limited to, trimethyl aluminum, titanium tetrachloride, and diethyl zinc; and wherein the second group includes, but is not limited to, ethylene glycol, propylene glycol, glycerol, hydroquinone, 1,2-ethanedithiol, and 1,3-propanedithiol.

As used herein, the term “ALE” refers to atomic layer etching, which is a thin film etching method. In certain embodiments, the term “thin” refers to a range of thickness from about 0.1 nm to about 300 nm. ALE uses a series of self-limiting chemical modification steps which only affect the top atomic layers of a substrate, and etching steps which remove only the chemically-modified areas, allowing for the removal of individual atomic layers.

As used herein, the term “instructional material” includes a publication, a recording, a diagram, or any other medium of expression that may be used to communicate the usefulness of the compositions and/or methods of the invention. In certain embodiments, the instructional material may be part of a kit useful for generating a composition of the invention. The instructional material of the kit may, for example, be affixed to a container that contains the compositions of the invention or be shipped together with a container that contains the compositions. Alternatively, the instructional material may be shipped separately from the container with the intention that the recipient uses the instructional material and the compositions cooperatively. For example, the instructional material is for use of a kit; instructions for use of the compositions; or instructions for use of the compositions.

As used herein, the term “μm” is the abbreviation for “micron” or “micrometer”, and it is understood that 1 μm=0.001 mm=10⁻⁶ m=1 millionth of a meter.

As used herein, the term “nanodevice” refers to a device that has at least one component with at least one spatial dimension less than 1 micron.

As used herein, the term “nm” is the abbreviation for “nanometer” and it is understood that 1 nm=1 nanometer=10⁻⁹ m=1 billionth of a meter.

As used herein, a “stable” compound refers to a compound or reaction product that does not spontaneously decompose or desorb from the surface of a substrate under reaction conditions.

As used herein, a “volatile” compound refers to a compound or reaction product that spontaneously desorbs from the surface of a substrate under reaction conditions.

Abbreviations used herein include: ALD, atomic layer deposition; ALE, atomic layer etching; atm, atmosphere, equivalent to 760 mmHg (torr); vs., versus.

Throughout this disclosure, various aspects of the invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range and, when appropriate, partial integers of the numerical values within ranges. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, and so on, as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.

Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently. Although the description herein contains many embodiments, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures, embodiments, claims, and examples described herein. Such equivalents were considered to be within the scope of this invention and covered by the claims appended hereto. For example, it should be understood, that modifications in reaction conditions, including but not limited to reaction times, reaction size/volume, and experimental reagents, such as solvents, catalysts, pressures, atmospheric conditions, e.g., nitrogen atmosphere, and reducing/oxidizing agents, with art-recognized alternatives and using no more than routine experimentation, are within the scope of the present application. In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. Any preceding definitions are provided to clarify their specific use in the context of the invention.

The following examples further illustrate aspects of the present invention. However, they are in no way a limitation of the teachings or disclosure of the present invention as set forth herein.

EXAMPLES

The invention is now described with reference to the following Examples. These Examples are provided for the purpose of illustration only, and the invention is not limited to these Examples, but rather encompasses all variations that are evident as a result of the teachings provided herein.

Materials and Methods

Atomic Layer Etching of TiN Films

All the reactions were conducted in a viscous flow, hot-wall reactor defined by a stainless steel tube with the length of ˜60 cm and inside diameter of 3.8 cm. The reactor was isothermally heated by heating elements attached on the outside of the reactor. The reaction temperatures from 150 to 350° C. were stabilized within ±0.04° C. of the set point by a proportional-integral-derivative (PID) temperature controller (2604, Eurotherm). A constant flow of 150 sccm UHP nitrogen gas (N₂, 99.999%, Airgas) was regulated by mass flow controllers (Type 1179A, MKS). This N₂ gas flow established a base pressure of ˜1 Torr in the reactor when pumped by a mechanical pump (Pascal 2015SD, Alcatel). The pressure was recorded by a capacitance manometer (Baratron 121A, MKS).

The demonstration of spontaneous etching of TiO₂ was performed using an in situ quartz crystal microbalance (QCM) located at the center of the hot-wall reactor. The gold-coated quartz crystal (polished, RC crystal, 6 MHz, Phillip Technologies) was placed in a sensor head (BSH-150, Inficon) and sealed with high temperature epoxy (Epo-Tek H21D, Epoxy technology). The mass changes during the reactions were measured by a thin film deposition monitor (Maxtek TM-400, Inficon). The TiO₂ films were deposited on the QCM crystal using TiO₂ ALD with titanium tetrachloride (TiCl₄, 99%, Strem) and deionized H₂O (Chromasolv for HPLC, Sigma-Aldrich) as the reactants at 200, 250, and 300° C. HF exposures on these TiO₂ ALD films were performed at the same temperature as the TiO₂ ALD reactions.

The HF source was a HF-pyridine solution (70 wt % HF, Sigma-Aldrich). A gold-plated stainless steel bubbler was filled with the HF-pyridine solution in a dry N₂-filled glove bag and held at room temperature. The HF vapor pressure of 90-100 Torr was in equilibrium with the HF-pyridine solution. Only HF vapor was delivered in the gas phase as confirmed by mass spectrometry. The pressure transients of HF were adjusted to ˜80 mTorr using a metering bellows-sealed valve (SS-4BMG, Swagelok).

The O₃ was produced in the corona discharge ozone generator (LG-14, Del Ozone). A flow of 480 sccm of UHP oxygen gas (O₂, 99.994%, Airgas) was supplied to produce a maximum ˜5 wt. % of O₃ in O₂. When the O₃ was not delivered to the reactor, the O₃ was passed through an ozone destruct unit (ODS-1, Ozone Solutions) connected to the exhaust line. Hydrogen peroxide (H₂O₂) vapor was evaporated with H₂O from a H₂O₂ solution (50 wt. %, Sigma Aldrich) contained in a glass bubbler. The H₂O₂ solution was held at room temperature.

The TiN films were prepared by SEMATECH. The TiN film was deposited on a 300 mm diameter silicon wafer by semiconductor ALD processes in commercially available tools. The initial thickness of TiN was measured as ˜70 Å by spectroscopic ellipsometry (SE) measurements. The TiN film on the silicon wafers was cleaved to samples that were 1.25 cm by 1.25 cm in size and used in ALE experiments. For the selectivity experiments, the Al₂O₃, HfO₂, ZrO₂, SiO₂, and Si₃N₄, thin films on silicon wafers were placed together with the TiN film in the reactor. The Al₂O₃, HfO₂ and ZrO₂ films were prepared by SEMATECH and deposited by ALD processes. The SiO₂ and Si₃N₄ films were prepared by SEMATECH and deposited by chemical vapor deposition (CVD) processes.

Al₂O₃ ALD films were coated inside the reactor before TiN ALE to minimize O₃ decomposition on the wall of the hot-wall reactor. The ALE reactions were conducted using a reaction sequence represented as x-30-y-30. This reaction sequence consists of an exposure of oxidation reagent for x s, 30 s of N₂ purge, y s of HF exposure and 30 s of N₂ purge. The ALE reactions using O₃ as an oxidation reagent were performed using an optimized reaction sequence of 3-30-1-30. The ALE reactions using H₂O, as an oxidation source were performed using an optimized reaction sequence of 2.5-30-1-30.

Ex situ spectroscopic ellipsometry (SE) measurements determined the thicknesses of the various films. Ψ and Δ at 240-900 nm were measured by a spectroscopic ellipsometer (M-2000, J. A. Woollam) and fitted by the analysis software (CompleteEASE, J. A. Woollam). These measurements employed an incidence angle of 75° near Brewster's angle of the silicon wafer. A Lorentz model was used for fitting the thickness of the TiN film. A Sellmeier model was used for fitting of the Al₂O₃, HfO₂, ZrO₂, SiO₂, and Si₃N₄ films.

The thicknesses, roughness, and film density were obtained by ex situ XRR analysis. The x-ray reflectivity (XRR) scans at 300-9000 arcsec were measured by a high resolution x-ray diffractometer (Bede D1, Jordan Valley Semiconductors) using Cu Kα radiation (λ=1.540 Å). The filament voltage was 40 kV and the current was 35 mA in the x-ray tube. All XRR scans were recorded with a 10 arcsec step size and a 5 s acquisition time and fitted using the analysis software (Bede REFS, Jordan Valley Semiconductors) to determine film thickness, film density and surface roughness.

Atomic Layer Etching of W and WO₃ Films

W samples were deposited on Si wafers with a 500 nm thermal oxide layer. The SiO₂ layer improves the sensitivity of the in situ spectroscopic ellipsometry (SE) analysis by providing interference enhancement. Al₂O₃ ALD films were first grown on the SiO₂ thermal oxide layer at 130° C. using 15 Al₂O₃ ALD cycles. These Al₂O₃ ALD films provided an adhesion layer for W ALD growth. W ALD films with a thickness of 250 Å were then deposited at 130° C. with sequential self-limiting reactions of WF₆ and Si₂H₆. These Al₂O₃ ALD and W ALD films were deposited in a separate hot-wall viscous flow reactor. Upon exposure to atmosphere, an oxide thickness of 12-30 Å is formed on the W film as determined by X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XRR) analysis.

The Si wafer was then diced to produce W coupons with dimensions of 1.6×1.6 cm. These W coupons were placed in a reaction chamber similar in design to plasma atomic layer deposition (ALD) reactors equipped for in situ SE measurements. The chamber walls were coated with ˜500 cycles of Al₂O₃ ALD using Al(CH₃)₃ and H₂O as the reactants at the chamber wall temperature of 170° C. WO₃ films were prepared by the oxidation of the W ALD films at 280° C. using an O₂ plasma at 600 W with an O₂ pressure of 100 mTorr. The O₂ plasma exposures produced a WO₃ film thickness of 130-150 Å on the W ALD films.

A remote inductively coupled plasma (ICP) provided oxygen radicals for the oxidation of W to WO₃. A quartz tube (6 cm inner diameter×25 cm long) encircled by a helical copper coil was the ICP source. A 13.56 MHz RF generator (Paramount RF Power Supply, Advanced Energy) and 50Ω impedance matching network (Navigator Digital Matching Network, Advanced Energy) were used in conjunction to generate the ICP plasma. The distance between the ICP source and the W coupon was ˜4 cm.

Etching of the WO₃ and W films was monitored by in situ SE using a J. A. Woollam M-2000D® ellipsometer having a spectral range of 240-1700 nm and using an incidence angle of 70°. The WO₃ and W films were analyzed to obtain film thicknesses after each reaction cycle or each individual reaction. A schematic showing the film stack and ellipsometer optical beams is shown in FIG. 2. Note that the individual layer thicknesses are not to scale. SE has the ability to measure the thicknesses of each individual layer in the film stack. This allows for simultaneous determination of the WO₃ and W film thicknesses.

The WO₃ films were analyzed using the COMPLETE EASE® software package (J. A. Woollam). The model employed a Tauc-Lorentz oscillator and a Gaussian oscillator. Only the parameters of the Tauc-Lorentz oscillator model were varied from the starting parameters to increase the accuracy of the model. The metal W layer underneath the WO₃ layer was measured by a B-Spline model. n and k values for bulk W were used as the initial parameters and were varied to fit the experimental data.

Boron trichloride (99.9%, Synquest Laboratories) and HF-pyridine (70 wt % HF, Sigma-Aldrich) were used as the reactants. The reactants were separately dosed into the reaction chamber together with a constant stream of ultrahigh purity (UHP) nitrogen. The reactants were introduced using two pneumatic valves (SWAGELOK®-HBVVCR4-C for BCl₃ or SWAGELOK®-6LVV-DPFR4-P-C for HF) on either side of a conductance limiting valve (Swagelok® SS-4BMG-VCR). The pneumatic valves were actuated using LABVIEW®. Between each reactant exposure, the reaction chamber was purged with UHP nitrogen gas for 130 s at a pressure of 1180 mTorr. The O₃ for the oxidation reaction during W ALE was produced by an O3ONIA® ozone generator with oxygen [Airgas, 99.999%]. The gas flow from the ozone generator contained ˜10% of O₃ in O₂. The O₂ pressure used for the oxidation reaction was 70 mTorr. Therefore, the O₃ pressure was ˜7 mTorr. The HF and BCl₃ purge times were 130 s during W ALE using the O₂/O₃ exposures. The purge time after the O₂/O₃ exposures was 60 s.

Samples were heated on a sample stage inside of the reaction chamber. A constant temperature of 207° C. was used for all of the ALE experiments performed to determine the self-limiting conditions. The temperature was initially targeted to be 200° C. A temperature calibration revealed that the temperature was 207° C. The temperature of the sample stage was varied during the studies of WO₃ ALE etch rate vs. temperature. The chamber walls were held constant at 170° C. A rotary vane pump (ALCATEL® 2010) was used to pump the chamber to a base pressure of ˜20 mTorr. A capacitance monometer measured the chamber base pressure and pressure transients from each reactant.

Example 1: Spontaneous Etching of TiO₂ by HF

FIG. 3 shows the mass changes during 10 consecutive HF exposures on TiO₂ films grown on the quartz crystal microbalance. The TiO₂ ALD films were deposited using TiCl₄ and H₂O as the reactants at 200, 250, and 300° C. Each HF exposure for 1 s leads to a mass loss consistent with spontaneous etching of the TiO₂ film. The average mass change during the HF exposure is −17 ng/cm² at 200° C. There are slightly higher mass changes at higher temperatures. Mass changes of −25 ng/cm² and −32 ng/cm² were obtained with the same HF exposure time of 1 s at 250° C. and 300° C., respectively. These results suggest that a HF exposure of 1 s is able to etch a TiO₂ thickness of 0.66-0.85 Å at 250-300° C. based on the density of 3.76 g/cm³ for TiO₂ ALD films.

The fluorination reaction given by Equation 3 has a reaction enthalpy ΔH=−22.7 kcal/mol at 25° C. This negative enthalpy value is maintained at high temperatures with ΔH=−21.9 kcal/mol at 250° C. Nearly constant negative ΔH values indicates the reaction is exothermic at all temperatures. The Gibbs free energy of this reaction is favorable at room temperature with ΔG=−6.1 kcal/mol at 25° C. The Gibbs free energy change becomes unfavorable at temperatures >150° C. for entropic reasons. The ΔG value at 250° C. is +6.3 kcal/mol. However, the ΔG values for the HF reaction with TiO₂ assumes equilibrium conditions. The etching reaction could be favorable under actual reaction conditions in the presence of excess of HF vapor when the H₂O reaction product is quickly removed by the N₂ purge gas. TiO₂+4HF TiF₄+2H₂O  (3)

Example 2: Thermal TiN ALE Using O₃ and HF

HF by itself or in conjunction other metal precursors cannot etch TiN. However, HF is able to etch TiO₂ spontaneously as shown in FIG. 3. These results suggest that if a TiO₂ layer can be formed on a TiN substrate using an oxidizing reagent, then a fluorination reagent, such as HF, should be able to etch the TiO₂ layer on the TiN substrate. These oxidation and fluorination reactions define the thermal ALE process illustrated in FIG. 1.

Ozone (O₃) is a gas phase reagent with strong oxidation power. O₃ has often been preferred to O₂ as an oxygen source for metal oxide ALD when its stronger oxidizing ability is needed or the H₂O purge is difficult. In addition, O₃ makes an excellent passivation oxide layer on polycrystalline aluminum metal or crystalline silicon. O₃ also can be employed as an oxidation reagent for the formation of a TiO₂ layer on the TiN substrate.

The proposed oxidation reaction of TiN using O₃ is given by Equation 2. This oxidation reaction is thermochemically favorable. The Gibbs free energy of this reaction is ΔG=−242.1 kcal/mol at 250° C. Other nitrogen-containing reaction products in addition to NO, such as N₂ and NO₂, may be possible. The ΔG values with N₂ or NO₂ as the reaction product yield favorable Gibbs free energy values of ΔG=−219 kcal/mol or −288 kcal/mol at 250° C., respectively. TiN+3O₃→TiO₂+NO+3O₂  (2)

The fluorination reaction of HF with TiO₂ is given by Equation 3. This reaction has ΔG values of −6.1 kcal/mol at 25° C. and +6.3 kcal/mol at 250° C. As discussed elsewhere herein, the positive ΔG value at 250° C. may not preclude the reaction because the reactions are not performed under equilibrium conditions. The production of TiF₄ leads to etching because TiF₄ is a volatile solid that sublimes at 284° C. After HF removes the TiO₂ layer on the TiN substrate, HF could also form nonvolatile TiF₃ by fluorination of TiN. The reaction between TiN and HF is given by: TiN(s)+3HF(g)→TiF₃(s)+NH₃(g)  (4) This reaction is favorable with a ΔG value of −37.0 kcal/mol at 250° C. TiF₃ is nonvolatile solid with a melting point of 1200° C. and boiling point of 1400° C.

FIG. 4 shows the spectroscopic ellipsometry (SE) and x-ray reflectivity (XRR) measurements of the initial TiN film thickness and the TiN film thickness after 25, 50, 100, 150, 200, and 300 ALE cycles using O₃ and HF as the reactants at 250° C. The film thickness vs. the number of ALE cycles is linear. An etch rate of 0.19 Å/cycle was obtained by SE using a least squares fitting. The XRR measurements on the same samples yield an etch rate of 0.20 Å/cycle. These XRR results are consistent with the SE results.

No TiN etch was observed when using O₂ by itself as the oxidation reactant together with HF. The oxidation reaction TiN(s)+3/2O₂(g)→TiO₂(s)+NO(g) is favorable having ΔG=−113.7 kcal/mol at 250° C. However, the oxidation of TiN by O₂ at <350° C. is known to be slow to form a thin TiO₂ layer. The surface of TiN is also known to form an intermediate titanium oxynitride (TiO_(x)N_(y)) phase in addition to a thin TiO₂ layer. The oxidation state of Ti in this intermediate phase was 3+ and 4+ by XPS analysis. The strong oxidizing ability of O₃ may be necessary for oxidation of TiN or TiO_(x)N_(y) to TiO₂. The TiO₂ layer can then be etched by HF during TiN ALE at 250° C.

FIGS. 5A-5B show the self-limiting behavior for the O₃ and HF exposures during TiN ALE at 250° C. The etch rates were obtained vs. reactant exposure time while holding the other reactant exposure time constant. The etch rate was determined using the average thickness change during 200 ALE cycles. FIG. 5A examines the self-limiting reaction using different O₃ exposure times with a HF exposure time of 1 s. A N₂ purge time of 30 s was used between reactant exposures. The etch rate increases and levels off at ˜0.2 Å/cycle vs. the O₃ exposure time. The oxidation of the TiN substrate is believed to be self-limiting because of the formation of a TiO₂ passivation layer.

FIG. 5B examines the self-limiting reaction using different HF exposure times with an O₃ exposure time of 3 s. The etch rate vs. the HF exposure time increases and levels off at ˜0.2 Å/cycle. The fluorination reaction is self-limiting because the etching reaction to produce volatile TiF₄ is limited by the thickness of the TiO₂ layer on the TiN substrate. HF does not etch the TiN film by itself. The HF is believed to form nonvolatile TiF₃ species on the TiN film. FIG. 3 shows that an HF exposure of 1 s is able to remove 25 ng/cm² or 0.66 Å of a TiO₂ film. This thickness of TiO₂ film removed by the HF exposure is much greater than the etch rate of 0.2 Å per cycle for the TiO₂ layer on the TiN substrate.

FIG. 6 shows results for TiN ALE in the presence of various materials. These results demonstrate the selective etching of TiN using O₃ and HF as the reactants at 250° C. There is a negligible thickness changes for Al₂O₃, HfO₂, ZrO₂, SiO₂, and Si₃N₄ during the TiN ALE reactions. The O₃ and HF exposures could not etch the Al₂O₃, HfO₂, and ZrO₂ metal oxides because the fluorides of these metal oxides, AlF₃, HfF₄, and ZrF₄, are stable and nonvolatile. The O₃ and HF exposures could also not etch silicon oxide and silicon nitride. In the absence of H₂O, dry HF cannot etch SiO₂. Previous thermal ALE studies using fluorination and ligand-exchange reactions could also not etch SiO₂ or Si₃N₄. New approaches are needed for SiO₂ ALE and Si₃N₄ ALE.

FIG. 7 shows XRR scans for the initial TiN film on the Si wafer and for TiN films after various numbers of ALE cycles with O₃ and HF as the reactants at 250° C. The initial TiN film thickness is reduced linearly after various numbers of ALE cycles as shown in FIG. 4. The XRR scans also reveal the roughness of the TiN films vs. the ALE cycles. The roughness of initial TiN film was 9.3 Å. The roughness was then reduced as the TiN film was smoothed by the ALE process. The etched TiN films have roughness values of 8.5 Å, 8.2 Å, and 7.5 Å after 25, 50, and 100 ALE cycles, respectively.

FIG. 8 shows the TiN ALE etch rates obtained at different temperatures using O₃ and HF as the reactants. The etch rates were obtained by the average of the thickness change measured by SE after 200 ALE cycles. All ALE reactions used an optimized reaction sequence of 3-30-1-30. The etch rate is 0.060 Å/cycle at 150° C. and increases to 0.20 Å/cycle at 250° C. The etch rates are not affected by temperature between 250-350° C. This result may indicate that a similar thickness for the TiO₂ layer is formed and removed by O₃ and HF at 250-350° C. The HF can only remove the amount of TiO₂ that is formed during the O₃ exposure. This constant etch rate over a range of temperatures is similar to the “ALD window” often observed for many ALD processes.

O₃ decomposition at high temperatures could affect the uniformity of the TiN film thicknesses in the reactor. To explore this issue, studies were performed after 200 ALE cycles at 250, 300 and 350° C. where the etch rate was nearly constant according to the results in FIG. 8. Five TiN film samples were placed within a length of 12.7 cm in the hot-wall reactor as shown in FIG. 9A. The TiN thicknesses were measured before and after the 200 ALE cycles to determine the uniformity of the TiN film.

FIG. 9A reveals that TiN ALE yielded fairly uniform TiN film thicknesses after 200 ALE cycles at 250° C. Positions 2-5 yield an etch rate of 0.19 Å/cycle. In contrast, a slightly larger etch rate of 0.27 Å/cycle is observed in Position 1 close to the reactant entry into the hot-wall reactor. This slightly larger etch rate may correspond with larger reactant exposures at Position 1.

FIG. 9B shows uniformity results obtained at a higher temperature of 350° C. These film thicknesses after 200 ALE cycles indicate that a lower etch rate is observed at the downstream positions furthest from the reactant entry into the hot-wall reactor. A recombination between gaseous O₃ molecules is negligible at a reaction pressure of ˜1 Torr. Consequently, this result may indicate that O₃ is undergoing some decomposition on the surfaces of the hot-wall reactor.

The decomposition of O₃ may be accelerated at the high temperature of 350° C. This decomposition may lead to lower O₃ concentrations downstream in the reactor. These lower O₃ concentrations then produce lower TiN etch rates and thicker TiN film thicknesses. Similar effects were observed for ZnO ALD as a function of position in the reactor using Zn(CH₂CH₃)₂ and O₃ as the reactants. Thinner ZnO ALD films were observed downstream in the reactor at 200° C. because of the lower O₃ concentrations.

Example 3: Thermal TiN ALE Using H₂O₂ and HF

Other oxidants in addition to O₃ may be able to oxidize the TiN substrate and form a TiO₂ layer. H₂O₂ is strong oxidizer that has been used as a reactant for metal oxide ALD. H₂O₂/H₂O vapor derived from 30-50 wt. % H₂O₂ solution has been used for ALD. H₂O₂ is stronger oxidizer than O₂, but weaker than O₃. The redox potential of oxidants measured in solution is consistent with this order of oxidant strength.

The proposed oxidation reaction of TiN using H₂O₂ can be expressed as: TiN(s)+3H₂O₂(g)→TiO₂(s)+NO(g)+3H₂O(g)  (4) This oxidation reaction is thermochemically favorable. The Gibbs free energy is ΔG=−211.6 kcal/mol at 250° C.

FIG. 10 shows the SE and XRR measurements of the initial TiN film thickness and the TiN film thickness after 50, 100, 200, and 400 ALE cycles using H₂O₂ and HF as the reactants at 250° C. The film thickness vs. the number of ALE cycles is linear. An etch rate of 0.15 Å/cycle was obtained by SE using a least squares fitting. The XRR measurements on the same samples yield an etch rate of 0.14 Å/cycle that is consistent with the SE results.

The ALE reactions using H₂O₂ and HF were also self-limiting. However, achieving reproducible results was challenging because of changes in the H₂O₂ concentration. Purging the excess H₂O after the H₂O₂/H₂O exposure was also difficult. There may be a thermal decomposition of H₂O₂ at high temperatures. These factors contributed to making H₂O₂ more problematic than O₃ as an oxidant for TiN ALE.

FIG. 11 shows the XRR scans of the initial TiN film on the Si wafer and the TiN films after various numbers of ALE cycles using H₂O₂ and HF as the reactants at 250° C. The initial TiN film thickness of 63 Å was reduced vs. number of ALE cycles as shown in FIG. 10. The initial TiN film roughness was 9.3 Å. This roughness was reduced gradually to 8.1 Å after 50 cycles and 6.5 Å after 100 ALE cycles. These measurements indicate that the TiN films were smoothed by TiN ALE using H₂O₂ and HF as the reactants. These results are consistent with the TiN film smoothing obtained from FIG. 7 during TiN ALE using O₃ and HF as the reactants.

Example 4: Generality of Thermal ALE Using Oxidation and Fluorination Reactions

The ALE of other materials including metal nitrides, metal sulfides, metal selenides, and elemental metals should be possible using sequential, self-limiting oxidation and fluorination reactions. This approach requires that oxidation by oxidants, such as O₃ and H₂O₂, be able to form a self-passivating metal oxide layer with a finite thickness that prevents further oxidation. The fluorination reactants, such as HF and SF₄, then remove only the metal oxide layer formed during the oxidation step. The removal occurs because the fluorides produced by the fluorination reactions are volatile. The oxygen in the metal oxide is removed as H₂O and SO₂ by HF and SF₄, respectively.

ALE processes using oxidation and fluorination reactions for representative materials are given in Table 1. The oxidation reactions for many transition metal nitrides that have volatile metal fluorides, such as TiN, to form their corresponding metal oxides are very favorable. The AG values for the oxidation of TiN, TaN, NbN, VN, W₂N, Mo₂N, MoN, and CrN using O₃ to form TiO₂, Ta₂O₅, Nb₂O₅, V₂O₅, WO₃, MoO₃, MoO₃, and CrO₃ are −241, −296, −284, −246, −294, −280, −298, and −238 kcal/mol at 250° C., respectively. The ΔG values for oxidation using H₂O₂ are somewhat lower but still very favorable with ΔG values<−200 kcal/mol.

The fluorination of these metal oxides should all yield volatile metal fluorides. The ΔG values for the fluorination of TiO₂, Ta₂O₅, Nb₂O₅, V₂O₅, WO₃, MoO₃, and CrO₃ using SF₄ to form TiF₄, TaF₅, NbF₅, VF₅, WF₆, MoF₆, and CrF₆ are −62, −79, −74, −45, −68, −53, and −40 kcal/mol at 250° C., respectively. These metal fluorides are all volatile. CrF₆ may rapidly convert to volatile CrF₅. For other metal nitrides that do not produce volatile fluorides after the fluorination of the metal oxide, a ligand-exchange reaction may be required to remove the metal fluoride.

Metal carbides are important materials that can be used as gate electrode materials with low work functions in semiconductor devices. Metal carbides have many similar properties to metal nitrides such as structure, bonding characteristics and electronic properties. The oxidation reactions for transition metal carbides to form their corresponding metal oxides are very favorable. The ΔG values for the oxidation of TaC, VC, and, WC using O₃ to form Ta₂O₅, V₂O₅ and WO₃ are −369, −321, −334 kcal/mol at 250° C., respectively. These reactions assume CO as an etching product. The Ta₂O₅, V₂O₅ and WO₃ metal oxides then should produce volatile fluorides during the fluorination reaction using HF or SF₄.

Transition metal sulfide and selenide are important 2D semiconductor materials. The oxidation of metal sulfides and selenides to form their corresponding metal oxides are also favorable. The ΔG values for the oxidation of WS₂, MoS₂, WSe₂, and MoSe₂ using O₃ to form WO₃, MoO₃, WO₃, MoO₃ are −553, −526, −531, and −515 kcal/mol at 250° C., respectively. The oxidation should also yield volatile SO₂ or SeO₃ reaction products. In addition to these thermochemical calculations, previous studies have recently reported the oxidation of WSe₂ to form a WO, layer on WSe₂. These metal oxides should then form volatile metal fluorides after fluorination with SF₄.

Elemental metals with volatile metal fluorides can also be etched using oxidation and fluorination reactions. The AG values for the oxidation of Ti, Ta, Nb, V, W, Mo, Cr using O₃ to form TiO₂, Ta₂O₅, Nb₂O₅, V₂O₅, WO₃, MoO₃, and CrO₃ are −288, −323, −305, −264, −296, −273, and −234 kcal/mol at 250° C., respectively. The oxidation reaction is expected to form a self-passivating metal oxide on the surface of the metal. If the oxide layer does not self-limit vs. oxidant exposure, other milder oxidation reactants such as NO₂, N₂O, and NO may be useful to obtain a self-limiting oxide thickness. These metal oxides should then form volatile metal fluorides after fluorination with HF or SF₄.

The choice of the fluorination reactant is important for metal ALE using oxidation and fluorination reactions. Some metals spontaneously etch with fluorination reactants that are stronger than HF. For example, XeF₂ and F₂ are both strong fluorination reagents. After removing the metal oxide, XeF₂ and F₂ could proceed to etch the underlying metal spontaneously. For example, XeF₂ spontaneously etches Ti, V, Nb, Ta, Ta, Mo, W, and metal alloys such as Ti/W. In contrast, HF can often etch the metal oxide layer without etching the underlying metal.

Some metals will also be difficult to etch using oxidation and fluorination reactions because their metal oxides are not favorable or stable. For example, Re and Pt have volatile fluorides such as ReF₆ and PtF₆, respectively. Re ALE may be difficult because the thermochemistry for fluorination of ReO₃ is not favorable. The reaction ReO₃(s)+3/2SF₄(g)→ReF₆(g)+3/2SO₂(g) has a ΔG value of +9 kcal/mol at 250° C. and only becomes favorable with a negative ΔG value at >450° C. Pt ALE may also be difficult because PtO₃ is not a stable oxide. The formation of PtO₂ on Pt may be possible but PtF₄ is not volatile.

TABLE 1 ALE Using Oxidation and Fluorination Reactions for Various Materials at 250° C. Metal Nitride Oxidation: TiN + 3O₃ → TiO₂ + NO + 3O₂ (ΔG = −241 kcal) Fluorination: TiO₂ + SF₄ → TiF₄ + SO₂ (ΔG = −62 kcal) Overall: TiN + 3O₃ + SF₄ → TiF₄ + SO₂ + 3O₂ + NO Metal Carbide Oxidation: NbC + 7/2O₃ → 1/2Nb₂O₅ + CO + 7/2O₂ (ΔG = −353 kcal) Fluorination: 1/2Nb₂O₅ + 5/4SF₄ → NbF₅ + 5/4SO₂ (ΔG = −74 kcal) Overall: NbC + 7/2O₃ + 5/4 SF₄ → NbF₅ + CO + 7/2O₂ + 5/4SO₂ Metal Sulfide Oxidation: WS₂ + 7O₃ → WO₃ + 2SO₂ + 7O₂ (ΔG = −552 kcal) Fluorination: WO₃ + 3/2SF₄ → WF₆ + 3/2SO₂ (ΔG = −68 kcal) Overall: WS₂ + 7O₃ + 3/2SF₄ → WF₆ + 7/2SO₂ + 7O₂ Metal Selenide Oxidation: MoSe₂ + 9O₃ → MoO₃ + 2SeO₃ + 9O₂ (ΔG = −519 kcal) Fluorination:: MoO₃ + 3/2SF₄ → MoF₆ + 3/2SO₂ (ΔG = −43 kcal) Overall: MoSe₂ + 9O₃ + 3/2SF₄ → MoF₆ + 2SeO₃ + 9O₂ + 3/2SO₂ Elemental Metal Oxidation: Ta + 5/2O₃ → 1/2Ta₂O₅ + 5/2O₂ (ΔG = −323 kcal) Fluorination: 1/2Ta₂O₅ + 5/4SF₄ → TaF₅ + 5/4SO₂ (ΔG = −79 kcal) Overall: Ta + 5/2O₃ + 5/4SF₄ → TaF₅ + 5/2O₂ + 5/4SO₂

Example 5: WO₃ ALE Using “Conversion-Fluorination” with BCl₃ and HF

FIG. 12 shows the WO₃ layer thickness measured using in situ SE during the etching of the WO₃ layer on the W film. These results were obtained using an AB exposure sequence with BCl3 and HF as the reactants. FIG. 12 displays results for 40 reaction cycles at a substrate temperature at 207° C. The initial WO₃ film thickness is □150 Å. The WO₃ film thickness decreases linearly until reaching a WO₃ film thickness of □10 Å. The etching of the WO₃ layer is linear over this range of thicknesses with an etch rate of 4.18 Å/cycle. The SE measurements confirmed that there was no change in the underlying W film thickness during WO₃ etching. The WO₃ etch rate is reduced dramatically when the WO₃ layer reaches a thickness of □10 Å. At this point, the WO₃/W interface is nearby and the WO₃ layer may undergo a transition to WO, oxides where x<3 before reaching the underlying W film. These WO, oxides may not be amenable to the “conversion-etch” procedure using BCl₃ and HF. As a result, the etch rate slows when the tungsten oxide layer thickness reaches a thickness of □5 Å. The underlying W film acts as an etch stop because the W film is not etched by the sequential exposures of BCl₃ and HF.

FIG. 13 shows the WO₃ thickness vs. number of BCl₃ and HF cycles at 207° C. for 20 cycles. The BCl₃ exposures were 325 mTorr s. The maximum BCl₃ pressure during these exposures was □40 mTorr. The HF exposures were 200 mTorr s. The maximum HF pressure during these exposures was □60 mTorr. Each dose was followed by an UHP N₂ purge lasting 130 s. The data points in FIG. 13 show the individual SE measurements of the WO₃ thickness after each reaction cycle.

The etching of WO₃ is linear with an R2 value of 0.999. The WO₃ etch rate is 4.18 Å/cycle. Multiple measurements of WO₃ ALE at 207° C. yielded etch rates that varied from 3.98-4.44 Å/cycle. Crystalline WO₃ has a monoclinic structure with cell dimensions of a=7.306 Å, b=7.540 Å, and c=7.692 Å. The WO₃ etch rate is slightly more than one-half of the WO₃ unit cell lengths. The etching of WO₃ by BCl₃ and HF occurs by the “conversion-fluorination” mechanism where BCl₃ converts the WO₃ surface layer to a B₂O₃ layer. During the conversion of WO₃ to B₂O₃, the likely reaction product is a volatile WO_(x)Cl_(y) compound. HF can then spontaneously remove the B₂O₃ layer by forming BF₃ and H₂O as reaction products. The removal of B₂O₃ regenerates the original WO₃ surface and completes one BCl₃/HF reaction cycle. The conversion of WO₃ to B₂O₃ is driven by the higher stability of B₂O₃ as compared to WO₃. The etching of B₂O₃ by HF occurs because both BF₃ and H₂O are volatile reaction products.

To prove that B₂O₃ can be spontaneously etched by HF, B₂O₃ films were grown with BCl₃ and H₂O at 20° C. This method was adapted from the previously reported B₂O₃ ALD process with BBr₃ and H₂O as the reactants at 20° C. B₂O₃ films were grown using 600 cycles of BCl₃ and H₂O at 20° C. This B₂O₃ ALD process led to B₂O₃ films with a thickness of □580 Å. X-ray photoelectron spectroscopy (XPS) analysis of these films was consistent with stoichiometric B₂O₃ films. The B₂O₃ films were then heated to 207° C. for the HF exposures. The B₂O₃ ALD films were exposed to HF pressures of 100 mTorr for 1 s. SE measurements were performed 60 s after the initial HF exposure. The SE measurements were then repeated 12 min after the first measurements to verify that no further etching occurred without additional HF exposures. FIG. 14 shows the B₂O₃ film thickness vs. the number of HF exposures for six HF exposures. Each HF exposure removes □2 Å of the B₂O₃ film. The second scan recorded after 12 min confirms that no additional etching is observed in the absence of HF exposures. These experiments demonstrate that HF can spontaneously etch the B₂O₃ film. The B₂O₃ etching is dependent on the HF pressure and the HF exposure time. The spontaneous B₂O₃ etching will continue for at least 20-30 HF exposures.

Both BCl₃ and HF exposures are required for WO₃ etching. Without HF exposures, BCl₃ exposures alone cannot etch WO₃. Without BCl₃ exposures, HF exposures alone did not etch WO₃. FIGS. 15A-15B examine the self-limiting BCl₃ and HF reactions during WO₃ ALE at 207° C. The etch rates were determined by varying one reactant while holding the other reactant at a constant reactant exposure. Each precursor was dosed into a stream of UHP N₂ gas at a pressure of 1180 mTorr. The purge time after each reactant exposure was 130 s. The self-limiting behavior for the WO₃ etch rate vs. the BCl₃ exposure is shown in FIG. 15A. The BCl₃ exposures were varied from 0 to 492 mTorr s. The HF exposures were held constant at 200 mTorr s. Increasing the BCl₃ exposure to >225 mTorr s did not produce significantly more WO₃ etching. The BCl₃ reaction with WO₃ is self-limiting at the larger BCl₃ exposures with an etch rate of □4.2 Å/cycle.

The self-limiting behavior of the HF exposure is shown in FIG. 15B. The HF exposure was varied from 0 to 318 mTorr s. BCl₃ exposures were held constant at 327 mTorr s. The WO₃ etch rate increases progressively vs. HF exposure. Increasing the HF exposure to >200 mTorr s did not produce significantly more WO₃ etching. The HF reaction with WO₃ is self-limiting at larger HF exposures with an etch rate of □4.2 Å/cycle.

FIG. 16 shows SE measurements that were recorded after each BCl₃ and HF exposure during WO₃ ALE at 207° C. for 26 half-cycles. The WO₃ thickness decreases linearly with the number of half-cycles. An expansion of the WO₃ thickness vs. number of half-cycles for four half-cycles is displayed in FIG. 17. The SE model assumed that the entire film was WO₃. Adding a Cauchy layer to account for the B₂O₃ layer on the WO₃ film after the BCl₃ conversion reaction did not improve the SE fitting. FIG. 17 indicates that the WO₃ thickness etched in one BCl₃/HF cycle is 4.29 Å. The thickness loss after the BCl₃ exposure is 2.99 Å. The thickness loss after the HF exposure is 1.30 Å.

The WO₃ thickness loss of 4.29 Å during one complete cycle represents 1.32×10⁻⁹ WO₃ mol/cm² based on a WO₃ density of 7.16 g/cm³ and a WO₃ molar mass of 231.8 g/mol. During the WO₃ etching, WO₃ can be converted to various amounts of B₂O₃ depending on the possible BCl₃ conversion reactions given by Equations 18, 19 and 20. The thickness loss of 2.99 Å after the BCl₃ exposure leaves a B₂O₃ thickness of 1.30 Å on the surface. The SE modeling assumes that the B₂O₃ thickness can be modeled as indistinguishable from a WO₃ thickness. This B₂O₃ thickness is then removed by the subsequent HF exposure. WO₃+2BCl₃(g)→B₂O₃+WCl₆(g) ΔG⁰(200° C.)=−7.7 kcal/mol WO₃+4/3BCl₃(g)→2/3B₂O₃+WCl₄(g) ΔG⁰(200° C.)=−4.3 kcal/mol  (18) WO₃+2/3BCl₃(g)→1/3B₂O₃+WO₂Cl₂(g)  (19) ΔG⁰(200° C.)=−7.8 kcal/mol  (20)

The B₂O₃ thickness of 1.30 Å on the surface prior to removal by HF is in agreement with the BCl₃ conversion reaction given by eq 5. This BCl₃ conversion reaction yields WO₂Cl₂ as the volatile reaction product. On the basis of eq 5, the predicted B₂O₃ thickness remaining on the surface after the conversion of 4.29 Å of WO₃ or 1.32×10⁻⁹ WO₃ mol/cm² is 1.25 Å. The predicted B₂O₃ thickness of 1.25 Å agrees well with the measured B2O3 thickness of 1.30 Å. Mass spectrometry studies are needed to confirm WO₂Cl₂ as the volatile reaction product.

The WO₃ film thicknesses vs. number of BCl₃ and HF reaction cycles at different substrate temperatures are shown in FIG. 18. The self-limiting reaction conditions at 207° C. were used for all of the various temperatures. The self-limiting conditions were a BCl₃ exposure of 325 mTorr s and an HF exposure of 200 mTorr s. All of the initial WO₃ thicknesses were referenced to a starting value of 140 Å to compare the results at 128, 160, 196, and 207° C. For all of the temperatures, the WO₃ etching is linear with the number of reaction cycles. The etch rate also increases at higher temperatures. The etch rates are 0.55, 2.04, 2.95, and 4.19 Å/cycle at 128, 160, 196, and 207° C., respectively.

FIG. 19 shows an Arrhenius plot of all of the etch rates at different temperatures acquired from the SE measurements. The approximately linear plot of ln(etch rate) vs. 1/T shows that the etch rates are nearly exponentially dependent on temperature. The temperature-dependent WO₃ etch rate exhibits an activation energy of 8.6 kcal/mol. The temperature dependence of the WO₃ etch rate provides a means to control the WO₃ etch rate.

Example 6: W ALE Using “Oxidation-Conversion-Fluorination” with O₃, BCl₃ and HF

The development of WO₃ ALE opens a pathway to W ALE. Tungsten can first be oxidized to WO₃. Subsequently, WO₃ can be etched using sequential BCl₃ and HF exposures as described in Example 5. The oxidation of W to produce WO₃ must be self-limiting to obtain atomic layer control of W etching. This requirement is demanding because oxidation of metals is generally very favorable, and often the oxidation can extend deep into the initial metal.

Tungsten oxidation has been studied extensively over a variety of temperatures and oxidation conditions. Many investigations have been reported at high temperatures >500° C. where tungsten oxidizes readily to form WO₃ and other tungsten oxides. At temperatures >800° C., these oxides can also desorb into the gas phase. At temperatures between 300 and 400° C., tungsten is oxidized by O₂ to form a WO₃ layer with thicknesses of 10-50 nm. The WO₃ layer acts as a diffusion barrier that limits further oxidation.

At lower temperatures of <300° C., the tungsten oxidation is limited to thin WO₃ films on the W substrate. Oxide thicknesses of 10-16 Å have been reported after O₂ exposures on polycrystalline W substrates for 1 h at 23-200° C. Somewhat thicker oxide thicknesses have been measured when using O₂ plasmas to oxidize tungsten at lower temperatures. Tungsten oxidation at these lower temperatures is compatible with the temperature for WO₃ ALE and also is self-limiting at thin film thicknesses that are required for an ALE process.

FIG. 20 shows results for the tungsten thickness vs. number of ABC reaction cycles during W ALE at 207° C. In these experiments, the surface of the W film was oxidized to WO₃ using O₂/O₃ pressures of 70 mTorr. The WO₃ layer was then etched using the next BCl₃ and HF exposures. The W ALE was conducted under self-limiting conditions for the O₂/O₃, BCl₃, and HF reactions at 207° C. as discussed below. The O₂/O₃ exposure was 3150 mTorr s, the BCl₃ exposure was 329 mTorr s, and the HF exposure was 2800 mTorr s.

FIG. 20 reveals that the tungsten film thickness decreases linearly vs. the number of cycles. The etch rate is 2.56 Å/cycle, and the R² value of the linear fit is 0.996. Multiple SE measurements of W ALE at 207° C. yielded etch rates that varied from 2.35-2.56 Å/cycle. XRR measurements confirmed the SE measurements. The W etch rate at 207° C. is slightly less than one unit cell length. W has a body-centered cubic structure with a unit cell length of 3.19 Å.

The WO₃ and W layer thicknesses could be determined simultaneously using SE measurements during W ALE. FIGS. 21A-21B show the concurrent SE measurements of the WO₃ and W film thicknesses vs. the number of half-cycles at 207° C. The half-cycles are the O₂/O₃ oxidation reaction and the BCl₃/HF etching reaction. The O₂/O₃ exposure was 3150 mTorr s, the BCl₃ exposure was 329 mTorr s, and the HF exposure was 2800 mTorr s. The ellipsometry measurements were performed after each half-cycle. The SE measurements were able to monitor both the growth and etching of the WO₃ layer and the concurrent removal of the underlying W film.

With an ABC exposure sequence, the WO₃ thickness is increased during W oxidization by O₂/O₃ and then decreased during the BCl₃ and HF etching reactions. The oxidation and etching leads to an oscillatory WO₃ thickness in FIG. 21A vs. the number of half-cycles. The increase in the WO₃ thickness from oxidation is 7.2 Å/half-cycle averaged over five random half-cycles after 12 half-cycles. The decrease in the WO₃ thickness from the BCl₃/HF etching is 7.7 Å/half-cycle averaged over five random half-cycles after 12 half-cycles.

The WO₃ thickness loss of ˜7.7 Å after the BCl₃/HF reactions during W ALE at 207° C. is larger than the WO₃ thickness loss of ˜4.2 Å/cycle during WO₃ ALE at 207° C. WO₃ ALE is performed with BCl₃ and HF as the reactants. W ALE is performed with O₂/O₃, BCl₃, and HF as the reactants. The addition of the O₂/O₃ exposure may alter the surface species and affect the BCl₃/HF reactions. Additional experiments were conducted where O₂/O₃ was removed from the ABC exposure sequence and WO₃ ALE was performed on WO₃ films produced using O₂/O₃. These experiments observed a WO₃ etch rate of ˜4.2 Å/cycle that is the same as the etch rate of ˜4.2 Å/cycle for WO₃ films produced using an O₂ plasma. The WO₃ etching is not dependent on the oxidant used to form the WO₃ film.

FIG. 21A also observes an increase in the WO₃ thickness from ˜30 to ˜40 Å during the first 10 half-cycles. This increase is followed by a reduction to an oxide thickness of ˜20 Å after 60 half-cycles. The change in the WO₃ thickness results from the competition between WO₃ growth during the O₂/O₃ exposures and WO₃ etching during the BCl₃/HF reactions. Higher or lower O₂/O₃ exposures were observed to result in more or less WO₃ growth. The presence of a maximum WO₃ thickness after 10 half-cycles in FIG. 21A may be related to nucleation effects combined with the competition between WO₃ growth and WO₃ etching.

Concurrent ellipsometry measurements of the W thickness are shown in FIG. 21B. While the WO₃ thickness is oscillating during the oxidation and etching half-cycles, the W thickness is reduced linearly vs. number of half-cycles. The W etching rate is 2.44 Å/cycle. A small oscillation of the W thickness was observed over the first 30 half-cycles. The decreases in the W thickness during the half-cycles occur after the O₂/O₃ exposures when W is oxidized to WO₃. This slight oscillation may be an artifact from the ellipsometric modeling.

An expansion of the oscillation in the WO₃ thickness in FIG. 21A is shown in FIG. 22. The increase of the WO₃ thickness during oxidation and the reduction of the WO3 thickness during etching for six consecutive half-cycles are dramatic. Values of the individual WO₃ thickness increases and decreases are given in FIG. 22. In addition, the increase in the WO₃ thickness from oxidation is 7.2 Å/half-cycle averaged over five random half-cycles after 12 half-cycles in FIG. 21A. The decrease in the WO₃ thickness from the BCl₃/HF etching is 7.7 Å/half-cycle averaged over five random half-cycles after 12 half-cycles in FIG. 21A. The ratio of the WO₃ thickness gain per half-cycle and the W thickness loss per cycle should be equal to the ratio of the WO₃ and W molar volumes. This expectation is based on conservation of tungsten mass where a W loss must equal a WO₃ gain. The ratio of the WO₃ gain and W loss is 7.2 (Å/half-cycle)/2.44 (Å/cycle)=2.95. In comparison, the ratio of the molar volumes for WO₃ and W is (32.4 cm³/mol)/(9.5 cm³/mol)=3.4.

The ratio of the molar volumes is only slightly higher than the ratio of the etch rates. This reasonable agreement is confirmation that W ALE occurs by conversion of W to WO₃ followed by the etching of WO₃. The slight differences in the ratios may also be explained by some WO₂ in the tungsten oxide layer. As compared to WO₃, WO₂ has a smaller molar volume of 20.0 cm³/mol. The molar volume of a mixture of WO₃/WO₂ would lower the ratio of the molar volumes for WO₃/WO₂ and W. The self-limiting nature of the O₂/O₃, BCl₃, and HF reactions is critical to establish a W ALE process. Self-limiting BCl₃ and HF reactions have already been established during the characterization of WO₃ ALE as shown in FIG. 15. The self-limiting behavior of the O₂/O₃, BCl₃, and HF reactions during W ALE also needs to be verified to confirm a self-limiting procedure for W ALE.

FIG. 23 displays the W etch rate vs. O₂/O₃ exposure during the ABC reaction sequence at 207° C. The pressure of the O₂/O₃ exposure was 70 mTorr for the different exposure times of 15, 30, 45, and 60 s. HF and BCl₃ exposures were held constant at 2800 and 500 mTorr s, respectively. The HF and BCl₃ pressures were 535 and 60 mTorr, respectively, during these exposures. FIG. 23 shows that the W etch rate increases with O₂/O₃ exposure and reaches a maximum etch rate of 2.45 Å/cycle at an O₂/O₃ exposure of 3150 mTorr s. The line fitting the data points is meant to guide the eye.

Results for the W etch rate vs. BCl₃ or HF exposure during W ALE at 207° C. are shown in FIGS. 24A and 24B, respectively. These measurements were performed by varying one reactant exposure and holding the other two reactant exposures constant. BCl₃ and HF were dosed into a stream of UHP N₂ that was at a pressure of 1180 mTorr. A purge of 130 s was employed after each reactant exposure. The O₂/O₃ exposure was 3150 mTorr s conducted at an O₂/O₃ pressure of 70 mTorr. A purge of 60 s was used after each O₂/O₃ exposure. FIG. 24A shows the results for varying the BCl₃ exposure while holding the HF and O₂/O₃ exposures constant at 2800 and 3150 mTorr s, respectively. The BCl₃ exposures were varied from 0 to 500 mTorr s. The BCl₃ exposure converts the WO₃ surface to a B₂O₃ surface layer. The B₂O₃ surface layer is then spontaneously etched by HF. The W etch rate increases rapidly with BCl₃ exposure. With a BCl₃ exposure of 329 mTorr s, the W etch rate reaches the self-limiting W etch rate of 2.45 Å/cycle. FIG. 24B shows the results for varying the HF exposure while holding the BCl₃ and O₂/O₃ exposures constant at 500 and 3150 mTorr s, respectively. Higher HF exposures progressively remove the B₂O₃ surface layer as volatile BF₃ and H₂O. With HF exposures of 2800 mTorr s, the W etch rate reaches the self-limiting W etch rate of 2.45 Å/cycle.

Additional experiments examined the oxidation of the W ALD films by successive O₂/O₃ exposures at 207° C. The O₂/O₃ exposures were 3150 mTorr s resulting from an O₂/O₃ pressure of 70 mTorr for 45 s. This is the same O₂/O₃ exposure that was employed in the W ALE experiments. The W ALD films had been exposed to atmosphere prior to loading into the reactor. The WO₃ thickness on the W ALD film was then measured by SE after each O₂/O₃ exposure. The WO₃ thickness vs. number of O₂/O₃ exposures is displayed in FIG. 25.

The initial WO₃ thickness in FIG. 25 is ˜33 Å. This thickness is close to the native oxide thicknesses on tungsten that have been measured earlier with XPS and XRR analysis. The WO₃ thickness is nearly constant at ˜33 Å after the first three O₂/O₃ exposures. This constant oxide thickness may be related to the O₂/O₃ exposure cleaning residual carbon from the surface. FIG. 25 then shows that the WO₃ thickness increases consistently after the third O₂/O₃ exposure. However, the increase is progressively reduced after each O₂/O₃ exposure. This behavior is suggestive of diffusion limited oxidation when the surface oxide acts as a barrier for oxidation of the underlying W metal. Similar behavior is also observed for the diffusion-limited oxidation of silicon described by the Deal-Grove model. The WO₃ thickness is ˜66 Å after 34 O₂/O₃ exposures. XPS analysis of the WO₃ film produced by these O₂/O₃ exposures was consistent with stoichiometric WO₃ with W in the 6+ oxidation state.

The larger WO₃ thicknesses produced by greater number of sequential O₂/O₃ exposures may lead to W etch rates that are higher after larger O₂/O₃ exposures. However, FIG. 25 shows that the W etch rate is self-limiting vs. O₂/O₃ exposure. This behavior suggests that the W ALE is self-limiting because of the self-limiting BCl₃ and HF reactions. Larger O₂/O₃ exposures may lead to larger WO₃ film thicknesses. However, the WO₃ removal is still limited by the self-limiting BCl₃ and HF reactions. The BCl₃ and HF reactions may only remove a fraction of the WO₃ film. After partial removal of the WO₃ film, the next O₂/O₃ exposure would then reestablish a larger WO₃ film thickness that is consistent with the O₂/O₃ exposure.

Additional experiments were performed at 207° C. by varying the O₂/O₃ exposure times with an O₂/O₃ pressure of 70 mTorr under self-limiting conditions for the BCl₃ and HF reactions. O₂/O₃ exposure times of 45 and 60 s both produced W ALE etch rates of 2.45 Å/cycle. However, the WO₃ film thickness was ˜20 Å for the 45 s O₂/O₃ exposures (3150 mTorr s) and ˜30 Å for the 60 s O₂/O₃ exposures (4200 mTorr s) after >20 reaction cycles. The consequence of larger O₂/O₃ exposure times at constant O₂/O₃ pressures is thicker WO₃ thicknesses during W ALE. However, the W ALE etch rates remain the same. These results argue that W ALE is self-limiting because of the self-limiting BCl₃ and HF reactions.

FIG. 21A shows that the WO₃ thickness is reduced to a thickness of ˜20 Å after 60 half-cycles during W ALE with an O₂/O₃ pressure of 70 mTorr and O₂/O₃ exposure time of 45 s under self-limiting conditions for the BCl₃ and HF reactions. Removal of this WO₃ layer on W may be important for applications where no oxide is desired for proper device function. This WO₃ layer can be removed by stopping the O₂/O₃ exposures and utilizing an AB reaction sequence with BCl₃ and HF exposures. FIG. 26A displays the removal of the WO₃ layer after W ALE vs. the number of BCl₃/HF reaction cycles using the self-limiting reaction conditions for WO₃ ALE at 207° C. The WO₃ layer thickness is reduced in thickness from 20 Å to a limiting thickness of ˜3 Å after >12 reaction cycles.

The corresponding W film thickness during the removal of the WO₃ layer is shown in FIG. 26B. While the WO₃ layer is removed during the AB reaction sequence, the W film thickness is nearly constant. The initial W film thickness is ˜218 Å. The W film thickness reaches 215-216 Å after >12 reaction cycles. These results indicate that the WO₃ layer on W can be removed with almost no effect on the W thickness. There are also alternative methods to remove the WO₃ layer based on hydrogen reduction using H₂ or H₂ plasma exposures.

The self-limiting conditions for the BCl₃ and HF exposures during WO₃ ALE at 207° C. were 325 and 200 mTorr s, respectively. In comparison, the self-limiting conditions for the O₂/O₃, BCl₃, and HF exposures during W ALE at 207° C. were 3150, 325, and 2800 mTorr s, respectively. The self-limiting conditions for the HF exposure are very different for WO₃ and W ALE. The inclusion of the O₂/O₃ exposure in the ABC reaction sequence for W ALE leads to a much larger selflimiting HF exposure. The chamber walls are coated with Al₂O₃ ALD, and HF is known to adsorb on Al₂O₃ at the chamber wall temperature of 170° C. The O₂/O₃ exposure may remove HF from the chamber walls. Larger HF exposures may then be required during the subsequent BCl₃/HF reaction to replace the adsorbed HF and obtain self-limiting behavior.

Example 7: Extension to Additional Materials

The “conversion fluorination” and “oxidation-conversion-fluorination” mechanisms can be useful for the ALE of a variety of additional metal materials. These mechanisms may prove to be valuable because some metal materials cannot be etched using the fluorination and ligand-exchange mechanism. Fluorination of many metal materials is not thermodynamically favorable with HF. However, stronger fluorination reactants, like SF₄ and XeF₂, can lead to volatile metal fluorides and spontaneous etching. The highly exothermic reaction of stronger fluorination reagents with metal materials can also lead to fluoride layers too thick for ALE. Some metal materials are also difficult to fluorinate because they do not have stable fluorides in the same oxidation state as the initial metal material. Other metal materials can be fluorinated but do not easily yield volatile products during the ligand-exchange reaction.

BCl₃ is important for the conversion of many initial metal oxides to B₂O₃ because B₂O₃ is a stable metal oxide and many metals have volatile chlorides or oxychlorides. Metals with volatile chlorides or oxychlorides include W, V, Nb, Ta, Cr, Mo, Fe, Au, Ga, Ge, Sn, As, Sb, Zr and Hf. The spontaneous etching of B₂O₃ by HF also plays a key role in the new conversion etching mechanisms. HF can spontaneously etch B₂O₃. However, HF cannot spontaneously etch many other metal oxides. Consequently, if the initial metal oxide can be converted to B₂O₃, then HF can spontaneously etch B₂O₃ and the underlying initial metal oxide will serve as an etch stop.

Table 2 explores the thermochemistry of a variety of conversion reactions for various metal oxides. Most of these metal oxides will have difficulty achieving self-limiting ALE using fluorination and ligand-exchange reactions. The reasons for the difficulty include: (1) formation of volatile fluorides that lead to spontaneous etching (WO₃, MoO₃, VO₂, V₂O₅, Ta₂O₅, GeO₂, As₂O₃, Au₂O₃, SbO₂, Sb₂O₃, and NbO₂); (2) lack of volatile products after ligand-exchange reaction with various metal precursors (Fe₂O₃); and (3) absence of a stable fluoride with the same oxidation state as the initial metal oxide (CrO₃). All of these metal oxides have volatile chlorides or oxychlorides and should be converted to B₂O₃ using BCl₃ based on thermochemical calculations. SnO₂, Ga₂O₃, ZrO₂, and HfO₂ ALE are possible using fluorination and ligand-exchange reactions. These metal oxides are included in Table 2 because they have volatile chlorides or oxychlorides and could be etched using the “conversion-fluorination” mechanism as an alternative.

The thermochemistry of most of these reactions in Table 2 is favorable as measured by the standard free energy changes. The examples in Table 2 use either BCl₃ or B(CH₃)₃ for the conversion reaction to form B₂O₃. The choice of BCl₃ or B(CH₃)₃ depends on the volatility of the reaction products. In addition, the ligand on the boron center can also lead to selective ALE. The thermochemical calculations reveal that Ga₂O₃ cannot be converted to B₂O₃ using B(CH₃)₃. Consequently, B(CH₃)₃ could be used to etch In₂O₃, GeO₂, As₂O₃, and SnO₂ without etching Ga₂O₃.

The conversion of metal oxides to other metal oxides besides B₂O₃ is also possible. One possibility is the conversion of metal oxides to TiO₂ using TiCl₄. Fluorination of TiO₂ using HF exposures would then spontaneously etch TiO₂ by producing volatile TiF₄ and H₂O reaction products. TiCl₄ may not be as useful as BCl₃ for conversion etch. The thermochemistry of conversion of metal oxides to TiO₂ using TiCl₄ is not as favorable as the thermochemistry of conversion of metal oxides to B₂O₃ using BCl₃. However, the conversion of the surface of a metal oxide to a TiO₂ surface layer may be useful for device applications to avoid a TiO₂ ALD processing step.

TABLE 2 Thermochemistry of a Variety of Conversion Reactions for Various Metal Oxides ΔG° at 150° C. (kcal) BCl₃ Conversion Reactions As₂O₃ + 2BCl₃ (g) → B₂O₃ + 2AsCl₃ (g) −81.3 Au₂O₃ + 2BCl₃ (g) → B₂O₃ + 2AuCl₃ (g) −134.5 CrO₃ + 2/3 BCl₃ (g) → 1/3 B₂O₃ + CrO₂Cl₂ (g) −93.7 Fe₂O₃ + 2BCl₃ (g) → B₂O₃ + 2FeCl₃ (g) −42.3 Ga₂O₃ + 2BCl₃ (g) → B₂O₃ + 2GaCl₃ (g) −61.6 GeO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + GeCl₄ (g) −76.7 HfO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + HfCl₄ (g) −15.6 MoO₃ + 2/3 BCl₃ (g) → 1/3 B₂O₃ + MoO₂Cl₂ (g) −19.1 NbO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + NbCl₄ (g) −13.6 Sb₂O₃ + 2BCl₃ (g) → B₂O₃ + 2SbCl₃ (g) −93.6 SnO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + SnCl₄ (g) −47.5 Ta₂O₅ + 10/3 BCl₃ (g) → 5/3 B₂O₃ + 2TaCl₅ (g) −42.8 VO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + VCl₄ (g) −26.0 V₂O₅ + 2BCl₃ (g) → B₂O₃ + 2VOCl₃ (g) −67.1 ZrO₂ + 4/3 BCl₃ (g) → 2/3 B₂O₃ + ZrCl₄ (g) −15.9 TiCl₄ Conversion Reactions As₂O₃ + 3/2 TiCl₃ (g) → 3/2 TiO₂ + 2AsCl₃ (g) −41.7 CrO₃ + 1/2 TiCl₃ (g) → 1/2 TiO₂ + CrO₂Cl₂ (g) −18.0 Fe₂O₃ + 3/2 TiCl₃ (g) → 3/2 TiO₂ + 2FeCl₃ (g) −2.7 Ga₂O₃ + 3/2 TiCl₃ (g) → 3/2 TiO₂ + 2GaCl₃ (g) −22.0 GeO₂ + TiCl₄ (g) → TiO₂ + GeCl₄ (g) −24.7 MoO₃ + 1/2 TiCl₃ (g) → 1/2 TiO₂ + MoO₂Cl₂ (g) −5.9 Sb₂O₃ + 3/2 TiCl₃ (g) → 3/2 TiO₂ + 2SbCl₃ (g) −54.0 SnO₂ + TiCl₄ (g) → TiO₂ + SnCl₄ (g) −21.1 B(CH₃)₃ Conversion Reactions As₂O₃ + 2B(CH₃)₃ (g) → B₂O₃ + 2As(CH₃)₃ (g) −83.0 Ga₂O₃ + 2B(CH₃)₃ (g) → B₂O₃ + 2Ga(CH₃)₃ (g) +34.1 GeO₂ + 4/3 B(CH₃)₃ (g) → 2/3 B₂O₃ + Ge(CH₃)₄ (g) −45.6 In₂O₃ + 2B(CH₃)₃ (g) → B₂O₃ + 2In(CH₃)₃ (g) −127.3 SnO₂ + 4/3 B(CH₃)₃ (g) → 2/3 B₂O₃ + Sn(CH₃)₄ (g) −26.8

The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention can be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims are intended to be construed to include all such embodiments and equivalent variations 

What is claimed is:
 1. A method of promoting atomic layer etching (ALE) on a solid substrate comprising a first metal compound, the method comprising: (a) contacting at least a portion of the surface of the solid substrate with an oxidant, whereby a first metal oxide is formed on the portion of surface of the solid substrate contacted with the oxidant; and (b) contacting at least a portion of the surface of the first metal oxide with a fluorinating agent, whereby a volatile metal fluoride is formed; whereby ALE of the solid substrate is promoted.
 2. The method of claim 1, wherein the first metal compound comprises at least one selected from the group consisting of metal boride, metal carbide, metal silicide, metal germanide, metal nitride, metal phosphide, metal arsenide, metal antimonide, metal sulfide, metal selenide, metal telluride, elemental metal, metal alloy, and hybrid organic-inorganic material.
 3. The method of claim 1, wherein the first metal compound is capable of forming a volatile metal fluoride.
 4. The method of claim 3, wherein the first metal fluoride has a melting point of less than 200° C. at 1 atm pressure.
 5. The method of claim 1, wherein the first metal compound comprises at least one species selected from the group consisting of Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr, Pt, B, Si, Ge, As, Sb, and Te.
 6. The method of claim 1, wherein the oxidant is at least one gaseous oxidant selected from the group consisting of O₂, H₂O₂, O₃, oxygen plasma, reactive oxygen radical species, NO₂ plasma, N₂O plasma, NO plasma, HNO₃ plasma, and SO₂ plasma.
 7. The method of claim 1, wherein the fluorinating agent is at least one gaseous fluorinating agent selected from the group consisting of HF, SF₄, XeF₂, F₂, and SF₆ plasma.
 8. The method of claim 1, wherein steps (a)-(b) are repeated one or more times, and wherein each repetition of steps (a)-(b) etches the solid substrate at a rate of about 0.1 Å to about 3.0 Å per cycle of steps (a)-(b).
 9. The method of claim 1, wherein the method is used for at least one process selected from patterning a surface of the substrate, cleaning a surface of the substrate, creating pores in a surface of the substrate, enlarging pores in a porous surface of the substrate, smoothing a surface of the substrate, removing defects from a surface of the substrate, removing damaged surface layers of the substrate, and removing unwanted clusters or islands from a surface of the substrate.
 10. The method of claim 1, wherein a film having a thickness greater than about 10 nm is etched by performing steps (a)-(b) one or more times, thereby generating a substrate film having a thickness of less than about 10 nm.
 11. A method of promoting atomic layer etching (ALE) on a solid substrate comprising a first metal compound, the method comprising: (a) contacting at least a portion of the surface of the solid substrate with an oxidant, whereby a first metal oxide is formed on the portion of surface of the solid substrate contacted with the oxidant; (b) contacting at least a portion of the surface of the first metal oxide with a second metal compound, whereby a second metal oxide is formed on the portion of surface of the first metal oxide contacted with the second metal compound and whereby a volatile first metal species is further formed; and (c) contacting at least a portion of the surface of the second metal oxide with a fluorinating agent, whereby a volatile metal fluoride is formed; whereby ALE of the solid substrate is promoted.
 12. The method of claim 11, wherein the first metal compound comprises at least one selected from the group consisting of metal boride, metal carbide, metal silicide, metal germanide, metal nitride, metal phosphide, metal arsenide, metal antimonide, metal sulfide, metal selenide, metal telluride, an elemental metal, metal alloy, and hybrid organic-inorganic material.
 13. The method of claim 11, wherein the first metal compound comprises a metal species capable of forming at least one selected from the group consisting of a volatile metal oxyfluoride, a volatile metal chloride, a volatile metal oxychloride, a volatile metal bromide, a volatile oxybromide, a volatile metal iodide, a volatile oxyiodide, and a volatile metal organic compound.
 14. The method of claim 11, wherein the first metal compound comprises a metal species capable of forming at least one second metal species selected from the group consisting of a metal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide, and metal oxyiodide, wherein the first metal species has a melting point of less than 200° C. at 1 atm pressure.
 15. The method of claim 11, wherein the second metal compound comprises a metal species capable of forming a stable metal oxide species on the surface of the solid substrate and at least one selected from the group consisting of a volatile metal fluoride, volatile metal oxyfluoride, volatile metal chloride, volatile metal oxychloride, volatile metal bromide, volatile metal oxybromide, volatile metal iodide, and volatile metal oxyiodide.
 16. The method of claim 11, wherein the second metal compound comprises a metal species capable of forming at least one second metal species selected from the group consisting of a metal fluoride, metal oxyfluoride, metal chloride, metal oxychloride, metal bromide, metal oxybromide, metal iodide, and metal oxyiodide, wherein the second metal species has a melting point of less than 200° C. at 1 atm pressure.
 17. The method of claim 11, wherein the first metal compound comprises at least one species selected from the group consisting of Al, Hf, Zr, Fe, Ni, Co, Mn, Mg, Rh, Ga, In, Zn, Pb, Cu, Cd, Hg, Ca, Li, Ag, Sn, Y, Ti, Ta, Nb, V, W, Mo, Ir, Ru, Rh, Re, Au, Cr, Pt, B, Si, Ge, As, Sb, and Te.
 18. The method of claim 11, wherein the second metal compound is at least one selected from BCl₃, B(CH₃)₃, BBr₃, BI₃, and TiCl₄.
 19. The method of claim 11, wherein the second metal oxide is B₂O₃ or TiO₂.
 20. The method of claim 11, wherein the oxidant is at least one gaseous oxidant selected from the group consisting of O₂, H₂O₂, O₃, oxygen plasma, reactive oxygen radical species, NO₂ plasma, N₂O plasma, NO plasma, HNO₃ plasma, and SO₂ plasma.
 21. The method of claim 11, wherein the fluorinating agent is at least one gaseous fluorinating agent selected from the group consisting of HF, SF₄, XeF₂, F₂, and SF₆ plasma.
 22. The method of claim 11, wherein the wherein steps (a)-(c) are repeated one or more times, and wherein each repetition of steps (a)-(c) etches the solid substrate at a rate of about 0.1 Å to about 3.0 Å per cycle of steps (a)-(c).
 23. The method of claim 11, wherein the method is used for at least one process selected from patterning a surface of the substrate, cleaning a surface of the substrate, creating pores in a surface of the substrate, enlarging pores in a porous surface of the substrate, smoothing a surface of the substrate, removing defects from a surface of the substrate, removing damaged surface layers of the substrate, and removing unwanted clusters or islands from a surface of the substrate.
 24. The method of claim 11, wherein a film having a thickness greater than about 10 nm is etched by performing steps (a)-(c) one or more times, thereby generating a substrate film having a thickness of less than about 10 nm. 